There are many allozyme polymorphisms with latitudinal clines, for which selection by climate is one obvious explanation. Both warm summer temperatures and cold winter temperatures could plausibly impose selection on enzyme polymorphisms, but because summer and winter temperatures are usually geographically correlated, there has been little attempt to use patterns of geographic variation to discriminate between these possible selective factors. The glucose-6-phosphate isomerase and mannose-6-phosphate isomerase allozyme polymorphisms in the amphipod crustacean Megalorchestia californiana have clines that are strongly associated with mean January air temperature but are only weakly associated with mean August air temperature. This suggests that winter temperatures are more likely than summer temperatures to be a selective factor on these polymorphisms.