diff --git a/R/findOverlaps-methods.R b/R/findOverlaps-methods.R
index 7a09f86..0c7288e 100644
--- a/R/findOverlaps-methods.R
+++ b/R/findOverlaps-methods.R
@@ -223,6 +223,118 @@ setMethod("findOverlaps", c("GenomicRanges", "GRangesList"),
     }
 )
 
+### - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
+### "findOverlaps" methods for GRangesFactor objects
+###
+
+.findOverlaps_Factor_other <- function(query, subject, 
+    maxgap=-1L, minoverlap=0L, type=c("any", "start", "end", "within", "equal"),
+    select=c("all", "first", "last", "arbitrary"), ignore.strand=FALSE)
+{
+    select <- match.arg(select)
+    type <- match.arg(type)
+    FUN <- function(Query, Select) {
+        findOverlaps(Query, subject, maxgap=maxgap, minoverlap=minoverlap,
+            type=type, select=Select, ignore.strand=ignore.strand)
+    }
+
+    if (length(query) < length(levels(query))) {
+        FUN(unfactor(query), Select=select)
+    } else {
+        if (select=="all") {
+            lev.hits <- FUN(levels(query), "all")
+            idx.hits <- findMatches(as.integer(query), queryHits(lev.hits))
+            Hits(from=queryHits(idx.hits), to=subjectHits(lev.hits)[subjectHits(idx.hits)],
+                nLnode=length(query), nRnode=length(subject), sort.by.query=TRUE)
+        } else {
+            lev.hits <- FUN(levels(query), select)
+            lev.hits[as.integer(query)]
+        }
+    }
+}   
+
+setMethod("findOverlaps", c("GRangesFactor", "GenomicRanges"), .findOverlaps_Factor_other)
+
+setMethod("findOverlaps", c("GRangesFactor", "GRangesList"), .findOverlaps_Factor_other)
+
+.findOverlaps_other_Factor <- function(query, subject,
+    maxgap=-1L, minoverlap=0L, type=c("any", "start", "end", "within", "equal"),
+    select=c("all", "first", "last", "arbitrary"), ignore.strand=FALSE)
+{
+    select <- match.arg(select)
+    type <- match.arg(type)
+    FUN <- function(Subject, Select) {
+        findOverlaps(query, Subject, maxgap=maxgap, minoverlap=minoverlap,
+            type=type, select=Select, ignore.strand=ignore.strand)
+    }
+
+    if (length(subject) < length(levels(subject))) {
+        FUN(unfactor(subject), select)
+    } else {
+        if (select=="all") {
+            lev.hits <- FUN(levels(subject), "all")
+            idx.hits <- findMatches(subjectHits(lev.hits), as.integer(subject))
+            Hits(from=queryHits(lev.hits)[queryHits(idx.hits)], to=subjectHits(idx.hits),
+                nLnode=length(query), nRnode=length(subject), sort.by.query=TRUE)
+        } else {
+            s.idx <- as.integer(subject)
+            if (select=="first") {
+                # Get the index of the first range for each level.
+                u <- which(!duplicated(s.idx))
+            } else {
+                # Get the index of the last range for each level.
+                u <- which(!duplicated(s.idx, fromLast=TRUE))
+            }
+            lev.hits <- FUN(levels(subject)[s.idx[u]], select)
+            u[lev.hits]
+        }
+    }
+}
+
+setMethod("findOverlaps", c("GenomicRanges", "GRangesFactor"), .findOverlaps_other_Factor)
+
+setMethod("findOverlaps", c("GRangesList", "GRangesFactor"), .findOverlaps_other_Factor)
+
+setMethod("findOverlaps", c("GRangesFactor", "GRangesFactor"), function(query, subject,
+    maxgap=-1L, minoverlap=0L, type=c("any", "start", "end", "within", "equal"),
+    select=c("all", "first", "last", "arbitrary"), ignore.strand=FALSE)
+{
+    if (length(query) < length(levels(query))) {
+        query <- unfactor(query)
+        callGeneric()
+    } else if (length(subject) < length(levels(subject))) {
+        subject <- unfactor(subject)
+        callGeneric()
+    } else {
+        FUN <- function(Query, Subject, Select) {
+            findOverlaps(Query, Subject, maxgap=maxgap, minoverlap=minoverlap, 
+                type=type, select=Select, ignore.strand=ignore.strand)
+        }
+
+        select <- match.arg(select)
+        if (select=="all") {
+            lev.hits <- FUN(levels(query), levels(subject), "all")
+            q.idx.hits <- findMatches(as.integer(query), queryHits(lev.hits))
+            s.idx.hits <- findMatches(subjectHits(lev.hits), as.integer(subject))
+            reconciler <- findMatches(subjectHits(q.idx.hits), queryHits(s.idx.hits))
+
+            Hits(from=queryHits(q.idx.hits)[queryHits(reconciler)], 
+                to=subjectHits(s.idx.hits)[subjectHits(reconciler)],
+                nLnode=length(query), nRnode=length(subject), sort.by.query=TRUE)
+        } else {
+            s.idx <- as.integer(subject)
+            if (select=="first") {
+                # Get the index of the first range for each level.
+                u <- which(!duplicated(s.idx))
+            } else {
+                # Get the index of the last range for each level.
+                u <- which(!duplicated(s.idx, fromLast=TRUE))
+            }
+            lev.hits <- FUN(levels(query), levels(subject)[s.idx[u]], select)
+            u[lev.hits][as.integer(query)]
+        }
+    }
+})
 
 ### =========================================================================
 ### findOverlaps-based methods
diff --git a/inst/unitTests/test_findOverlaps-methods.R b/inst/unitTests/test_findOverlaps-methods.R
index b479919..c26d2ab 100644
--- a/inst/unitTests/test_findOverlaps-methods.R
+++ b/inst/unitTests/test_findOverlaps-methods.R
@@ -318,6 +318,97 @@ test_findOverlaps_with_circular_sequences <- function()
     .checkHits(1:4, 1:4, 4, 4, current5, select="all")
 }
 
+test_findOverlaps_with_GRangesFactors <- function() {
+    ir0 <- IRanges(c(5, 25, 20, 30, 45, 35, 10, 15), width=10)
+    gr0 <- GRanges("chrA", ir0)
+    F0 <- Factor(gr0[rep(seq_along(gr0), seq_along(gr0))])
+
+    ir1 <- IRanges(c(18, 8, 28, 38), width=5)
+    gr1 <- GRanges("chrA", ir1)
+    F1 <- Factor(gr1[rep(seq_along(gr1), rev(seq_along(gr1)))])
+
+    ######################
+    # findOverlaps works with a Factor as the query.
+    out <- findOverlaps(F0, gr1)
+    ref <- findOverlaps(unfactor(F0), gr1)
+    checkIdentical(out, ref)
+
+    out <- findOverlaps(F0, gr1, minoverlap=4)
+    ref <- findOverlaps(unfactor(F0), gr1, minoverlap=4)
+    checkIdentical(out, ref)
+
+    # findOverlaps works with a Factor as the subject.
+    out <- findOverlaps(gr0, F1)
+    ref <- findOverlaps(gr0, unfactor(F1))
+    checkIdentical(sort(out), sort(ref)) # hack to overcome lack of subject sorting guarantees.
+
+    out <- findOverlaps(gr0, F1, maxgap=4)
+    ref <- findOverlaps(gr0, unfactor(F1), maxgap=4)
+    checkIdentical(sort(out), sort(ref)) # hack to overcome lack of subject sorting guarantees.
+
+    # findOverlaps works with two Factors.
+    out <- findOverlaps(F0, F1)
+    ref <- findOverlaps(unfactor(F0), unfactor(F1))
+    checkIdentical(out, ref)
+
+    out <- findOverlaps(F0, F1, maxgap=2)
+    ref <- findOverlaps(unfactor(F0), unfactor(F1), maxgap=2)
+    checkIdentical(out, ref)
+
+    ######################
+    # All methods work with different settings for 'select'.
+
+    out <- findOverlaps(F0, gr1, select="first")
+    ref <- findOverlaps(unfactor(F0), gr1, select="first")
+    checkIdentical(out, ref)
+
+    out <- findOverlaps(F0, gr1, select="last")
+    ref <- findOverlaps(unfactor(F0), gr1, select="last")
+    checkIdentical(out, ref)
+
+    out <- findOverlaps(gr0, F1, select="first")
+    ref <- findOverlaps(gr0, unfactor(F1), select="first")
+    checkIdentical(out, ref)
+
+    out <- findOverlaps(gr0, F1, select="last")
+    ref <- findOverlaps(gr0, unfactor(F1), select="last")
+    checkIdentical(out, ref)
+
+    out <- findOverlaps(F0, F1, select="first")
+    ref <- findOverlaps(unfactor(F0), unfactor(F1), select="first")
+    checkIdentical(out, ref)
+
+    out <- findOverlaps(F0, F1, select="last")
+    ref <- findOverlaps(unfactor(F0), unfactor(F1), select="last")
+    checkIdentical(out, ref)
+
+    ######################
+    # All methods work correctly with small Factors that cause unfactor()ing.
+    out <- findOverlaps(F0[1:2], gr1)
+    ref <- findOverlaps(unfactor(F0[1:2]), gr1)
+    checkIdentical(out, ref)
+
+    out <- findOverlaps(F0[3], gr1)
+    ref <- findOverlaps(unfactor(F0[3]), gr1)
+    checkIdentical(out, ref)
+
+    out <- findOverlaps(gr0, F1[4:3])
+    ref <- findOverlaps(gr0, unfactor(F1[4:3]))
+    checkIdentical(sort(out), sort(ref))
+
+    out <- findOverlaps(gr0, F1[1])
+    ref <- findOverlaps(gr0, unfactor(F1[1]))
+    checkIdentical(sort(out), sort(ref))
+
+    out <- findOverlaps(F0[10:13], F1)
+    ref <- findOverlaps(unfactor(F0[10:13]), unfactor(F1))
+    checkIdentical(out, ref)
+
+    out <- findOverlaps(F0, F1[2])
+    ref <- findOverlaps(unfactor(F0), unfactor(F1[2]))
+    checkIdentical(out, ref)
+}
+
 test_poverlaps <- function() {
     ans <- poverlaps(GRanges(), GRanges())
     checkIdentical(ans, Rle())
diff --git a/man/findOverlaps-methods.Rd b/man/findOverlaps-methods.Rd
index be2e737..7467f50 100644
--- a/man/findOverlaps-methods.Rd
+++ b/man/findOverlaps-methods.Rd
@@ -6,6 +6,9 @@
 \alias{findOverlaps,GRangesList,GenomicRanges-method}
 \alias{findOverlaps,GenomicRanges,GRangesList-method}
 \alias{findOverlaps,GRangesList,GRangesList-method}
+\alias{findOverlaps,GRangesFactor,GenomicRanges-method}
+\alias{findOverlaps,GenomicRanges,GRangesFactor-method}
+\alias{findOverlaps,GRangesFactor,GRangesFactor-method}
 
 \alias{countOverlaps}
 \alias{countOverlaps,GenomicRanges,GenomicRanges-method}
@@ -97,6 +100,10 @@
   For \code{type="equal"} with GRangesList objects, \code{query[[i]]}
   matches \code{subject[[j]]} iff for each range in \code{query[[i]]}
   there is an identical range in \code{subject[[j]]}, and vice versa.
+
+  If either or both \code{query} or \code{subject} are \link{GRangesFactor}
+  objects, overlaps are identified based on the unique levels. This improves
+  the efficiency of this function for large GRangesFactors with few levels.
 }
 
 \value{