diff --git a/ChangeLog b/ChangeLog
index 5ce6a305d8..9eac1dd169 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Sat Aug  1 05:18:36 2009  NARUSE, Yui  <naruse@ruby-lang.org>
+
+	* string.c (tr_trans): can't use singlebyte optimization when
+	  the replacement is multibyte. [ruby-core:24612]
+
 Fri Jul 31 18:01:34 2009  NAKAMURA Usaku  <usa@ruby-lang.org>
 
 	* lib/securerandom.rb (SecureRandom.random_bytes): return string should
diff --git a/string.c b/string.c
index 34f35b8b29..6e6db0053a 100644
--- a/string.c
+++ b/string.c
@@ -4871,7 +4871,8 @@ tr_trans(VALUE str, VALUE src, VALUE repl, int sflag)
 	    if (r == errc) r = trrepl.now;
 	    if (c < 256) {
 		trans[c] = r;
-		if (r > 255) singlebyte = 0;
+		if (r > 255 || (r > 127 && rb_enc_mbmaxlen(enc) != 1))
+		    singlebyte = 0;
 	    }
 	    else {
 		if (!hash) hash = rb_hash_new();