diff --git a/setup.py b/setup.py
index b4df58aae1..adff3d5b05 100644
--- a/setup.py
+++ b/setup.py
@@ -433,6 +433,7 @@ def build_swarm():
       install_requires=['numpy >= 1.9.0',
                         'scipy >= 0.14.0',
                         'cogent == 1.5.3',
+                        'natsort < 4.0.0',
                         'matplotlib >= 1.1.0, != 1.4.2',
                         'pynast == 1.2.2', 'qcli >= 0.1.1, < 0.2.0', 'gdata',
                         'biom-format >= 2.1.4, < 2.2.0',