diff --git a/requirements.optional.txt b/requirements.optional.txt
index 1e1bc44..eb7bdaa 100644
--- a/requirements.optional.txt
+++ b/requirements.optional.txt
@@ -17,7 +17,7 @@ bpforms
 openbabel < 3
 
 [bio]
-biopython == 1.77
+biopython >= 1.75, < 1.78
 
 [revisioning]
 gitpython