diff --git a/VERSIONS b/VERSIONS
index 98ba8ce4..888853f6 100644
--- a/VERSIONS
+++ b/VERSIONS
@@ -6,7 +6,8 @@ Note that not every commit will be logged here; that is what the Github commit h
 
 
 development head (in the master branch):
-
+	fixed a bug in calcWattersonsTheta(): the value calculated would be incorrect if a window (start/end) was specified
+	
 
 version 4.2.2 (Eidos version 3.2.2):
 	fix SLiMgui crash with code completion
diff --git a/core/slim_functions.cpp b/core/slim_functions.cpp
index 486f779e..e340fe68 100644
--- a/core/slim_functions.cpp
+++ b/core/slim_functions.cpp
@@ -272,6 +272,8 @@ R"V0G0N({
 		species = community.allSpecies;
 	}
 	
+	length = species.chromosome.lastPosition + 1;
+	
 	if (isNULL(muts))
 		muts = species.mutations;
 	
@@ -297,7 +299,7 @@ R"V0G0N({
 	k = size(muts);
 	n = genomes.size();
 	a_n = sum(1 / 1:(n-1));
-	theta = (k / a_n) / (species.chromosome.lastPosition + 1);
+	theta = (k / a_n) / length;
 	return theta;
 })V0G0N";