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Analyses of geographic variation in the skull of meadow voles (Microtus pennsylvanicus) indicate that phenetic distances among samples are not related to geographic distance: a minimum spanning tree based on average taxonomic distance superimposed on a map of 38 localities provides no particular phenetic clustering of those samples geographically proximate. A multiple regression of phenetic component one (skull size) onto orthogonally rotated climatic factors explains much less morphometric variation (25.6%) than a simple correlation with recorded extreme low temperature (38.9%). Multiple regression of phenetic principal component two (interorbital width) onto the same climatic factors explains minimally more morphological variation (42.1%) than a simple correlation with mean annual number of days with frost (41.7%). Microtus pennsylvanicus shows a pattern of size variation that is the reverse of Bergmann's rule: these voles are large where it is warm and small where it is cold. Since small size reduces total energy expenditure, we predict that during times of extreme low temperature (i) smaller voles will be less energetically stressed than larger voles and (ii) large size will be actively selected against.