diff --git a/tests/printenv.py b/tests/printenv.py
--- a/tests/printenv.py
+++ b/tests/printenv.py
@@ -39,14 +39,15 @@
        if k.startswith("HG_") and v]
 env.sort()
 
-out.write("%s hook: " % name)
+out.write(b"%s hook: " % name.encode('ascii'))
 if os.name == 'nt':
     filter = lambda x: x.replace('\\', '/')
 else:
     filter = lambda x: x
-vars = ["%s=%s" % (k, filter(v)) for k, v in env]
-out.write(" ".join(vars))
-out.write("\n")
+vars = [b"%s=%s" % (k.encode('ascii'), filter(v).encode('ascii'))
+        for k, v in env]
+out.write(b" ".join(vars))
+out.write(b"\n")
 out.close()
 
 sys.exit(exitcode)