diff --git a/common.go b/common.go
index ca763b9..456ed38 100644
--- a/common.go
+++ b/common.go
@@ -17,11 +17,12 @@ const (
 
 // randStr returns a string of length n constructed
 // from pseudo-randomly selected characters from t.
-func randStr(n int, t string) string {
+// The pseudo-randomness uses random values from s.
+func randStr(n int, t string, s rand.Source) string {
 	l := len(t) - 1
 	chars := make([]byte, n)
 	if l > 0 {
-		for i, p := range rand.New(rand.NewSource(1238)).Perm(n) {
+		for i, p := range rand.New(s).Perm(n) {
 			chars[i] = t[p%l]
 		}
 	}
diff --git a/common_test.go b/common_test.go
index aec89f2..5a24403 100644
--- a/common_test.go
+++ b/common_test.go
@@ -4,29 +4,33 @@
 
 package epi
 
-import "testing"
+import (
+	"math/rand"
+	"testing"
+)
 
 func TestRandStr(t *testing.T) {
 	for _, test := range []struct {
 		n int
 		t string
+		s rand.Source
 	}{
-		{1, ""},
-		{1, "a"},
-		{1, "abc"},
-		{10, "ab"},
-		{10, "abcdefghijklmnopqrstuvwxyz"},
-		{100, "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ"},
+		{1, "", rand.NewSource(1)},
+		{1, "a", rand.NewSource(2)},
+		{1, "abc", rand.NewSource(3)},
+		{10, "ab", rand.NewSource(4)},
+		{10, "abcdefghijklmnopqrstuvwxyz", rand.NewSource(5)},
+		{100, "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ", rand.NewSource(6)},
 	} {
-		if got := len(randStr(test.n, test.t)); got != test.n {
-			t.Errorf("len(randStr(%d, %q) = %d; want %d", test.n, test.t, got, test.n)
+		if got := len(randStr(test.n, test.t, test.s)); got != test.n {
+			t.Errorf("len(randStr(%d, %q, %v) = %d; want %d", test.n, test.t, test.s, got, test.n)
 		}
 	}
 }
 
 func benchRandStr(b *testing.B, size int) {
 	for i := 0; i < b.N; i++ {
-		randStr(size, "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ")
+		randStr(size, "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ", rand.NewSource(int64(i)))
 	}
 }
 
diff --git a/dutchflag_test.go b/dutchflag_test.go
index f338a85..279046e 100644
--- a/dutchflag_test.go
+++ b/dutchflag_test.go
@@ -36,7 +36,7 @@ func benchRearrange(b *testing.B, size int) {
 	p := size / 3
 	q := size
 	for i := 0; i < b.N; i++ {
-		data := rand.New(rand.NewSource(1234)).Perm(size)
+		data := rand.New(rand.NewSource(int64(i))).Perm(size)
 		j := data[i%size]%(q-p) + p
 		b.StartTimer()
 		Rearrange(data, j)
diff --git a/maxdiff_test.go b/maxdiff_test.go
index 4871cc8..172d1f5 100644
--- a/maxdiff_test.go
+++ b/maxdiff_test.go
@@ -44,7 +44,7 @@ func TestMinBatteryCap(t *testing.T) {
 func benchMinBatteryCap(b *testing.B, size int) {
 	b.StopTimer()
 	for i := 0; i < b.N; i++ {
-		data := rand.New(rand.NewSource(1235)).Perm(size)
+		data := rand.New(rand.NewSource(int64(i))).Perm(size)
 		b.StartTimer()
 		MinBatteryCap(data)
 		b.StopTimer()
diff --git a/nextperm_test.go b/nextperm_test.go
index 823b39a..c85b086 100644
--- a/nextperm_test.go
+++ b/nextperm_test.go
@@ -34,7 +34,7 @@ func TestNext(t *testing.T) {
 func benchNext(b *testing.B, size int) {
 	b.StopTimer()
 	for i := 0; i < b.N; i++ {
-		data := rand.New(rand.NewSource(1236)).Perm(size)
+		data := rand.New(rand.NewSource(int64(i))).Perm(size)
 		b.StartTimer()
 		NextPerm(data)
 		b.StopTimer()
diff --git a/powerset_test.go b/powerset_test.go
index 6765438..6cd8e22 100644
--- a/powerset_test.go
+++ b/powerset_test.go
@@ -62,7 +62,7 @@ func benchPowerSet(b *testing.B, size int) {
 	b.StopTimer()
 	for i := 0; i < b.N; i++ {
 		data := make([]interface{}, size)
-		for j, p := range rand.New(rand.NewSource(1237)).Perm(size) {
+		for j, p := range rand.New(rand.NewSource(int64(i))).Perm(size) {
 			data[j] = p
 		}
 		b.StartTimer()
diff --git a/reversewords_test.go b/reversewords_test.go
index bd560a3..0c1a26b 100644
--- a/reversewords_test.go
+++ b/reversewords_test.go
@@ -4,7 +4,10 @@
 
 package epi
 
-import "testing"
+import (
+	"math/rand"
+	"testing"
+)
 
 func TestReverseWords(t *testing.T) {
 	for _, test := range []struct {
@@ -31,7 +34,7 @@ func TestReverseWords(t *testing.T) {
 func benchReverseWords(b *testing.B, size int) {
 	b.StopTimer()
 	for i := 0; i < b.N; i++ {
-		s := randStr(size, "The quick brown fox jumps over the lazy dog") // Pangram.
+		s := randStr(size, "The quick brown fox jumps over the lazy dog", rand.NewSource(int64(i))) // Pangram.
 		b.StartTimer()
 		ReverseWords(s)
 		b.StopTimer()
diff --git a/rlecompr_test.go b/rlecompr_test.go
index 5160631..ac95d9d 100644
--- a/rlecompr_test.go
+++ b/rlecompr_test.go
@@ -4,7 +4,10 @@
 
 package epi
 
-import "testing"
+import (
+	"math/rand"
+	"testing"
+)
 
 func TestRLEEncode(t *testing.T) {
 	for _, test := range []struct {
@@ -40,7 +43,7 @@ func TestRLEEncode(t *testing.T) {
 func benchRLEEncode(b *testing.B, size int) {
 	b.StopTimer()
 	for i := 0; i < b.N; i++ {
-		s := randStr(size, "abcdefghijklmnopqrstuvwxyz")
+		s := randStr(size, "abcdefghijklmnopqrstuvwxyz", rand.NewSource(int64(i)))
 		b.StartTimer()
 		RLEEncode(s)
 		b.StopTimer()
@@ -87,7 +90,7 @@ func TestRLEDecode(t *testing.T) {
 func benchRLEDecode(b *testing.B, size int) {
 	b.StopTimer()
 	for i := 0; i < b.N; i++ {
-		s, ok := RLEEncode(randStr(size, "abcdefghijklmnopqrstuvwxyz"))
+		s, ok := RLEEncode(randStr(size, "abcdefghijklmnopqrstuvwxyz", rand.NewSource(int64(i))))
 		if !ok {
 			b.Errorf("RLEEncode did not encode string properly")
 		}
diff --git a/spiralmetrix_test.go b/spiralmetrix_test.go
index 64f8e26..23b1b92 100644
--- a/spiralmetrix_test.go
+++ b/spiralmetrix_test.go
@@ -51,7 +51,7 @@ func TestClockwise(t *testing.T) {
 func benchClockwise(b *testing.B, size int) {
 	b.StopTimer()
 	for i := 0; i < b.N; i++ {
-		ints := rand.New(rand.NewSource(1239)).Perm(size)
+		ints := rand.New(rand.NewSource(int64(i))).Perm(size)
 		data := make([][]int, len(ints))
 		for j := range data {
 			data[j] = append([]int(nil), ints...)