diff --git a/ObsoleteFiles.inc b/ObsoleteFiles.inc
index 715f73b7d638..2f70ebe2bec1 100644
--- a/ObsoleteFiles.inc
+++ b/ObsoleteFiles.inc
@@ -38,6 +38,8 @@
 #   xargs -n1 | sort | uniq -d;
 # done
 
+# 20170319: io_test requires zh_TW.Big5 locale.
+OLD_FILES+=usr/tests/lib/libc/locale/io_test
 # 20170319: remove nls for non supported Big5* locales
 OLD_DIRS+=usr/share/nls/zh_HK.Big5HKSCS
 OLD_DIRS+=usr/share/nls/zh_TW.Big5
diff --git a/lib/libc/tests/locale/Makefile b/lib/libc/tests/locale/Makefile
index e05cbae3ac9f..83caf8cb3bd3 100644
--- a/lib/libc/tests/locale/Makefile
+++ b/lib/libc/tests/locale/Makefile
@@ -20,7 +20,8 @@ ATF_TESTS_C+=		wcsrtombs_test
 ATF_TESTS_C+=		wcstombs_test
 ATF_TESTS_C+=		wctomb_2_test
 
-NETBSD_ATF_TESTS_C=	io_test
+# Note: io_test requires zh_TW.Big5 locale (see ^/head@r315568)
+#NETBSD_ATF_TESTS_C=	io_test
 NETBSD_ATF_TESTS_C+=	mbrtowc_test
 NETBSD_ATF_TESTS_C+=	mbstowcs_test
 NETBSD_ATF_TESTS_C+=	mbsnrtowcs_test