diff --git a/R/XStringSet-io.R b/R/XStringSet-io.R
index e594f71b..13a6fdf5 100644
--- a/R/XStringSet-io.R
+++ b/R/XStringSet-io.R
@@ -362,8 +362,11 @@ readAAStringSet <- function(filepath, format="fasta",
         stop(wmsg("'width' must be a single integer"))
     if (!is.integer(width))
         width <- as.integer(width)
-    if (width < 1L)
-        stop(wmsg("'width' must be an integer >= 1"))
+    if (width < 1L) {
+        ## set to maximum possible width
+        ## using `lengths` because the function overrides `width`
+        width <- max(lengths(x))
+    }
     lkup <- get_seqtype_conversion_lookup(seqtype(x), "B")
     .Call2("write_XStringSet_to_fasta",
           x, filexp_list, width, lkup,
diff --git a/man/XStringSet-io.Rd b/man/XStringSet-io.Rd
index 1f389842..bea1cb7d 100644
--- a/man/XStringSet-io.Rd
+++ b/man/XStringSet-io.Rd
@@ -169,7 +169,7 @@ saveXStringSet(x, objname, dirpath=".", save.dups=FALSE, verbose=TRUE)
 
     If \code{format="fasta"}, the \code{width} argument can be used to
     specify the maximum number of letters per line of sequence.
-    \code{width} must be a single integer.
+    \code{width} must be a single integer, and uses an unbounded width if set to a value less than 1.
 
     If \code{format="fastq"}, the \code{qualities} argument can be used to
     specify the quality strings. \code{qualities} must be a \link{BStringSet}
diff --git a/src/read_fasta_files.c b/src/read_fasta_files.c
index b480060b..52d4c1fc 100644
--- a/src/read_fasta_files.c
+++ b/src/read_fasta_files.c
@@ -628,6 +628,7 @@ SEXP write_XStringSet_to_fasta(SEXP x, SEXP filexp_list, SEXP width, SEXP lkup)
 {
 	XStringSet_holder X;
 	int x_length, width0, lkup_len, i, j1, j2, dest_nbytes;
+	int nmax_write_bytes, nwrote_bytes;
 	const int *lkup0;
 	SEXP filexp, x_names, desc;
 	Chars_holder X_elt;
@@ -636,8 +637,7 @@ SEXP write_XStringSet_to_fasta(SEXP x, SEXP filexp_list, SEXP width, SEXP lkup)
 	x_length = _get_length_from_XStringSet_holder(&X);
 	filexp = VECTOR_ELT(filexp_list, 0);
 	width0 = INTEGER(width)[0];
-	if (width0 >= IOBUF_SIZE)
-		error("'width' must be <= %d", IOBUF_SIZE - 1);
+	nmax_write_bytes = width0 >= IOBUF_SIZE ? IOBUF_SIZE - 1 : width0;
 	iobuf[width0] = 0;
 	if (lkup == R_NilValue) {
 		lkup0 = NULL;
@@ -657,21 +657,33 @@ SEXP write_XStringSet_to_fasta(SEXP x, SEXP filexp_list, SEXP width, SEXP lkup)
 		}
 		filexp_puts(filexp, "\n");
 		X_elt = _get_elt_from_XStringSet_holder(&X, i);
-		for (j1 = 0; j1 < X_elt.length; j1 += width0) {
-			j2 = j1 + width0;
+		j1 = 0;
+		nwrote_bytes = 0;
+		while(j1 < X_elt.length){
+			j2 = j1 + nmax_write_bytes;
 			if (j2 > X_elt.length)
 				j2 = X_elt.length;
 			dest_nbytes = j2 - j1;
+			if(nwrote_bytes + dest_nbytes > width0){
+				// align write to size width0
+				dest_nbytes = width0 - nwrote_bytes;
+				j2 = j1 + dest_nbytes;
+			}
 			j2--;
 			Ocopy_bytes_from_i1i2_with_lkup(j1, j2,
 				iobuf, dest_nbytes,
 				X_elt.ptr, X_elt.length,
 				lkup0, lkup_len);
 			iobuf[dest_nbytes] = 0;
+			nwrote_bytes += dest_nbytes;
+			j1 = j2+1;
 			filexp_puts(filexp, iobuf);
-			filexp_puts(filexp, "\n");
+			if(j1 == X_elt.length || nwrote_bytes == width0){
+				// add newline if at end of line (width0) or end of sequence
+				filexp_puts(filexp, "\n");
+				nwrote_bytes = 0;
+			}
 		}
 	}
 	return R_NilValue;
 }
-
diff --git a/src/read_fastq_files.c b/src/read_fastq_files.c
index 61770577..a1133cb7 100644
--- a/src/read_fastq_files.c
+++ b/src/read_fastq_files.c
@@ -615,9 +615,25 @@ static void write_FASTQ_id(SEXP filexp, const char *markup, const char *id)
 	filexp_puts(filexp, "\n");
 }
 
-static void write_FASTQ_seq(SEXP filexp, const char *buf)
+static void write_FASTQ_seq(SEXP filexp, const Chars_holder X,
+		const int *lkup0, int lkup_len)
 {
-	filexp_puts(filexp, buf);
+	// write sequences in chunks in case sequence is longer than I/O buffer
+	int i1, i2, bytes_to_write, bytes_remaining;
+	bytes_remaining = X.length;
+	i1 = 0;
+	while(bytes_remaining){
+		bytes_to_write = bytes_remaining >= IOBUF_SIZE ? IOBUF_SIZE-1 : bytes_remaining;
+		i2 = i1+bytes_to_write-1;
+		Ocopy_bytes_from_i1i2_with_lkup(i1, i2,
+			iobuf, bytes_to_write,
+			X.ptr, X.length,
+			lkup0, lkup_len);
+		iobuf[bytes_to_write] = 0;
+		filexp_puts(filexp, iobuf);
+		i1 = i2 + 1;
+		bytes_remaining -= bytes_to_write;
+	}
 	filexp_puts(filexp, "\n");
 }
 
@@ -677,18 +693,8 @@ SEXP write_XStringSet_to_fastq(SEXP x, SEXP filexp_list,
 	for (i = 0; i < x_length; i++) {
 		id = get_FASTQ_rec_id(x_names, q_names, i);
 		X_elt = _get_elt_from_XStringSet_holder(&X, i);
-		if (X_elt.length >= IOBUF_SIZE)
-			error("XStringSet object (or derivative) to "
-			      "write 'x' cannot contain strings\n  longer "
-			      "than %d ('x[[%d]]' has %d characters)",
-			      IOBUF_SIZE - 1, i + 1, X_elt.length);
-		Ocopy_bytes_from_i1i2_with_lkup(0, X_elt.length - 1,
-			iobuf, X_elt.length,
-			X_elt.ptr, X_elt.length,
-			lkup0, lkup_len);
-		iobuf[X_elt.length] = 0;
 		write_FASTQ_id(filexp, FASTQ_line1_markup, id);
-		write_FASTQ_seq(filexp, iobuf);
+		write_FASTQ_seq(filexp, X_elt, lkup0, lkup_len);
 		write_FASTQ_id(filexp, FASTQ_line3_markup, id);
 		if (qualities != R_NilValue) {
 			write_FASTQ_qual(filexp, X_elt.length, &Q, i);
diff --git a/tests/testthat/test-XStringSet-io.R b/tests/testthat/test-XStringSet-io.R
index 40c52551..e502a547 100644
--- a/tests/testthat/test-XStringSet-io.R
+++ b/tests/testthat/test-XStringSet-io.R
@@ -175,3 +175,43 @@ test_that("XStringSet read/write is correct", {
     unlink(tmpfilefq)
 })
 
+test_that("Writing XStringSets larger than IOBUF_SIZE functions correctly", {
+    ## IOBUF_SIZE is 200,003
+    tf <- tempfile()
+    set.seed(456L)
+    x1 <- paste(sample(DNA_BASES, 400020, replace=TRUE), collapse='')
+    x2 <- paste(sample(DNA_BASES, 200127, replace=TRUE), collapse='')
+    xss <- DNAStringSet(c(x1, x2))
+    names(xss) <- c("seq1", "seq2")
+    exp_output_fasta <- paste0('>seq1', x1, '>seq2', x2)
+    for(w in c(5, 80, 213, 200003, width(xss)[1], width(xss)[2], -1)){
+      writeXStringSet(xss, tf, width = w)
+      opt <- readLines(tf)
+      opt <- paste(opt, collapse='')
+      expect_equal(opt, exp_output_fasta)
+      read_seqs <- readDNAStringSet(tf)
+      expect_true(all(as.character(read_seqs) == as.character(xss)))
+    }
+})
+
+test_that("Writing QualityScaledXStringSets larger than IOBUF_SIZE functions correctly", {
+    ## IOBUF_SIZE is 200,003
+    tf <- tempfile()
+    set.seed(456L)
+    x1 <- paste(sample(DNA_BASES, 200127, replace=TRUE), collapse='')
+    x2 <- paste(sample(DNA_BASES, 400020, replace=TRUE), collapse='')
+    q1 <- paste(sample(as.character(1:9), 200127, replace=TRUE), collapse='')
+    q2 <- paste(sample(as.character(1:9), 400020, replace=TRUE), collapse='')
+    xss <- DNAStringSet(c(x1, x2))
+    quals <- PhredQuality(c(q1, q2))
+    names(xss) <- c("seq1", "seq2")
+    names(quals) <- c("seq1", "seq2")
+    exp_output_fastq <- paste0("@seq1", x1, "+seq1", q1, "@seq2", x2, "+seq2", q2)
+    qss <- QualityScaledDNAStringSet(xss, quals)
+    writeQualityScaledXStringSet(qss, tf)
+    opt <- paste(readLines(tf), collapse='')
+    expect_equal(opt, exp_output_fastq)
+    read_seqs <- readQualityScaledDNAStringSet(tf)
+    expect_true(all(as.character(read_seqs) == as.character(xss)))
+    expect_true(all(as.character(quality(read_seqs)) == as.character(quals)))
+})