diff --git a/python/python3-mpmath/README b/python/python3-mpmath/README
index a74dd4742b..5d1fe568a9 100644
--- a/python/python3-mpmath/README
+++ b/python/python3-mpmath/README
@@ -4,3 +4,5 @@ by Fredrik Johansson since 2007, with help from many contributors.
 
 This is the python3 version of mpmath and it can coexist with SBo's
 mpmath package.
+
+python3-gmpy2 is an optional runtime dependency.