diff --git a/doc/src/sgml/regress.sgml b/doc/src/sgml/regress.sgml
index 6a3ac95558..59083eca1a 100644
--- a/doc/src/sgml/regress.sgml
+++ b/doc/src/sgml/regress.sgml
@@ -194,6 +194,48 @@ gmake standbycheck
    Additional automated testing may be available in later releases.
   </para>
   </sect2>
+
+  <sect2>
+   <title>Locale and encoding</title>
+
+   <para>
+    By default, the tests against a temporary installation use the
+    locale defined in the current environment and the corresponding
+    database encoding as determined by <command>initdb</command>.  It
+    can be useful to test different locales by setting the appropriate
+    environment variables, for example:
+<screen>
+gmake check LANG=C
+gmake check LC_COLLATE=en_US.utf8 LC_CTYPE=fr_CA.utf8
+</screen>
+    For implementation reasons, setting <envar>LC_ALL</envar> does not
+    work for this purpose; all the other locale-related environment
+    variables do work.
+   </para>
+
+   <para>
+    When testing against an existing installation, the locale is
+    determined by the existing database cluster and cannot be set
+    separately for the test run.
+   </para>
+
+   <para>
+    You can also choose the database encoding explicitly by setting
+    the variable <envar>MULTIBYTE</envar>, for example:
+<screen>
+gmake check LANG=C MULTIBYTE=EUC_JP
+</screen>
+    Setting the database encoding this way typically only makes sense
+    if the locale is C; otherwise the encoding is chosen automatically
+    from the locale, and specifying an encoding that does not match
+    the locale will result in an error.
+   </para>
+
+   <para>
+    The encoding can be set for tests against a temporary or an
+    existing installation.
+   </para>
+  </sect2>
   </sect1>
 
   <sect1 id="regress-evaluation">
diff --git a/src/test/regress/GNUmakefile b/src/test/regress/GNUmakefile
index cb67e5a12a..15b9ec49f7 100644
--- a/src/test/regress/GNUmakefile
+++ b/src/test/regress/GNUmakefile
@@ -23,9 +23,6 @@ endif
 # where to find psql for testing an existing installation
 PSQLDIR = $(bindir)
 
-# default encoding
-MULTIBYTE = SQL_ASCII
-
 # maximum simultaneous connections for parallel tests
 MAXCONNOPT =
 ifdef MAX_CONNECTIONS
@@ -141,7 +138,7 @@ tablespace-setup:
 ## Run tests
 ##
 
-pg_regress_call = ./pg_regress --inputdir=$(srcdir) --dlpath=. --multibyte=$(MULTIBYTE) $(NOLOCALE)
+pg_regress_call = ./pg_regress --inputdir=$(srcdir) --dlpath=. $(if $(MULTIBYTE),--multibyte=$(MULTIBYTE)) $(NOLOCALE)
 
 check: all tablespace-setup
 	$(pg_regress_call) --temp-install=./tmp_check --top-builddir=$(top_builddir) --schedule=$(srcdir)/parallel_schedule $(MAXCONNOPT) $(TEMP_CONF)