diff --git a/.bumpversion.cfg b/.bumpversion.cfg
index 74393353d..254387d6b 100644
--- a/.bumpversion.cfg
+++ b/.bumpversion.cfg
@@ -1,5 +1,5 @@
 [bumpversion]
-current_version = 3.20.dev
+current_version = 3.21.dev
 commit = True
 tag = False
 parse = (?P<major>\d+)\.(?P<minor>\d+)\.(?P<release>[a-z]+)?(?P<patch>\d+)?
diff --git a/setup.py b/setup.py
index b3be794d4..58f7480aa 100644
--- a/setup.py
+++ b/setup.py
@@ -8,7 +8,7 @@
 
 # Static version number which is updated by bump2version
 # Do not change this manually - use 'bump2version <major/minor/patch/release>'
-__version_tag__ = "3.20.dev"
+__version_tag__ = "3.21.dev"
 
 setup_kwargs = {
     "name": "dxtbx",