diff --git a/t/Makefile.am b/t/Makefile.am
index 7c1d3db..09d6f9b 100644
--- a/t/Makefile.am
+++ b/t/Makefile.am
@@ -1,18 +1,27 @@
 AM_CPPFLAGS = -I$(top_srcdir)/lib
 LIBPSORT = $(top_builddir)/lib/libptsort.a
 
-if WITH_CRYB_TEST
+check_PROGRAMS=
+dist_check_SCRIPTS =
+EXTRA_DIST =
 
-check_PROGRAMS = t_aa
+if WITH_CRYB_TEST
+check_PROGRAMS += t_aa
 t_aa_LDADD = $(LIBPSORT) $(LIBCRYB_TEST)
-
 endif
 
-dist_check_SCRIPTS = t_dp_red.sh
+dist_check_SCRIPTS += t_dp_red.sh
+EXTRA_DIST += t_dp_red.ti
+EXTRA_DIST += t_dp_red.to
 
-${dist_check_SCRIPTS}: t_subr.sh
+dist_check_SCRIPTS += t_parallel.sh
+EXTRA_DIST += t_parallel.ti
+EXTRA_DIST += t_parallel-dp.to
+EXTRA_DIST += t_parallel-PDd.to
+EXTRA_DIST += t_parallel-Pp.to
+EXTRA_DIST += t_parallel-P.to
 
-EXTRA_DIST = t_dp_red.ti t_dp_red.to
+${dist_check_SCRIPTS}: t_subr.sh
 
 TESTS = $(check_PROGRAMS) $(dist_check_SCRIPTS)
 
diff --git a/t/t_parallel-P.to b/t/t_parallel-P.to
new file mode 100644
index 0000000..208deac
--- /dev/null
+++ b/t/t_parallel-P.to
@@ -0,0 +1,4 @@
+r
+b1 a1 c1
+b2 a2
+b3 a3
diff --git a/t/t_parallel-PDd.to b/t/t_parallel-PDd.to
new file mode 100644
index 0000000..a0a2ca7
--- /dev/null
+++ b/t/t_parallel-PDd.to
@@ -0,0 +1,4 @@
+      3 r
+      2 b1 a1
+      1 b2 a2
+      0 c1 b3 a3
diff --git a/t/t_parallel-Pp.to b/t/t_parallel-Pp.to
new file mode 100644
index 0000000..52780fb
--- /dev/null
+++ b/t/t_parallel-Pp.to
@@ -0,0 +1,4 @@
+      3 r
+      2 b1 a1 c1
+      1 b2 a2
+      0 b3 a3
diff --git a/t/t_parallel-dp.to b/t/t_parallel-dp.to
new file mode 100644
index 0000000..27bb4f8
--- /dev/null
+++ b/t/t_parallel-dp.to
@@ -0,0 +1,8 @@
+      3       3 r
+      2       2 b1
+      2       2 a1
+      0       2 c1
+      1       1 b2
+      1       1 a2
+      0       0 b3
+      0       0 a3
diff --git a/t/t_parallel.sh b/t/t_parallel.sh
new file mode 100755
index 0000000..06ee229
--- /dev/null
+++ b/t/t_parallel.sh
@@ -0,0 +1,59 @@
+#!/bin/sh
+#-
+# Copyright (c) 2017-2024 Dag-Erling Smørgrav
+# All rights reserved.
+#
+# Redistribution and use in source and binary forms, with or without
+# modification, are permitted provided that the following conditions
+# are met:
+# 1. Redistributions of source code must retain the above copyright
+#    notice, this list of conditions and the following disclaimer.
+# 2. Redistributions in binary form must reproduce the above copyright
+#    notice, this list of conditions and the following disclaimer in the
+#    documentation and/or other materials provided with the distribution.
+# 3. The name of the author may not be used to endorse or promote
+#    products derived from this software without specific prior written
+#    permission.
+#
+# THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
+# ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+# IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+# ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
+# FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+# DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
+# OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+# HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+# LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+# OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+# SUCH DAMAGE.
+#
+
+. ./t_subr.sh
+
+ptsort() {
+	local options="$1"
+	local ifile="${abs_srcdir}/t_parallel.ti"
+	local ofile="${abs_srcdir}/t_parallel$options.to"
+	local tfile="${abs_builddir}/t_parallel$options.t$$"
+
+	grep '^[^#]' "${ifile}" |
+	    "${abs_top_builddir}/bin/ptsort" $options ${_verbose:+"-v"} >"${tfile}"
+
+	if cmp -s "${ofile}" "${tfile}" ; then
+		ok parallel: ptsort $options
+	else
+		if [ -n "${_verbose}" ] ; then
+			diff --side-by-side --width 80 "${ofile}" "${tfile}" >&2
+		fi
+		not_ok parallel: ptsort $options
+	fi
+
+	rm -f "${tfile}"
+}
+
+ptsort -P
+ptsort -PDd
+ptsort -Pp
+ptsort -dp
+
+tap
diff --git a/t/t_parallel.ti b/t/t_parallel.ti
new file mode 100644
index 0000000..0b0fd4b
--- /dev/null
+++ b/t/t_parallel.ti
@@ -0,0 +1,8 @@
+r	a1
+a1	a2
+a2	a3
+r	b1
+b1	b2
+b2	b3
+r	c1
+c1	2