diff --git a/Makefile b/Makefile
--- a/Makefile
+++ b/Makefile
@@ -5,7 +5,18 @@
 # % make PREFIX=/opt/ install
 
 export PREFIX=/usr/local
+
+# Default to Python 3.
+#
+# But there are still unresolved issues with Python 3 on Windows. See
+# discussion in https://phab.mercurial-scm.org/D7258. So we keep things
+# on `python.exe` there (which may or may not be Python 3).
+ifeq ($(OS),Windows_NT)
 PYTHON?=python
+else
+PYTHON?=python3
+endif
+
 $(eval HGROOT := $(shell pwd))
 HGPYTHONS ?= $(HGROOT)/build/pythons
 PURE=