Postnatal growth and development in the little brown bat, Myotis lucifugus, was characterized for two latitudinally separated populations to examine effects of the length of the growing season . Bats from New Hampshire (43°N) and bats experiencing a shorter and colder growing season in Alberta (54°N), were compared with respect to birth size and growth rate (as measured by forearm length and body mass), flight development, ontogeny of thermoregulation, body composition, and the allometric growth of body parts. Alberta bats were shown to grow more slowly, and flight development was somewhat delayed. However, Alberta bats were born larger, suggesting that when growth is slowed by colder growing conditions, selection favors greater prenatal investment allowing Alberta juveniles to reach the size required to survive the first hibernal period. Thermoregulatory development is delayed in Alberta juveniles, indicating that these bats defer investment in thermogenesis until a critical point in postnatal growth . A model 1s presented defining this critical point by relating thermogenic ability and conductance to the timing of onset of homeothermy. Analysis of body composition revealed that differences between the two populations were primarily manifested in the proportions of lean dry mass and water. Fat contributed more to energy density among New Hampshire bats, suggesting that Alberta bats allocate more energy to structural development than energy storage during early growth. These results, coupled with evidence of delayed reproduction in Alberta bats, indicate that growth 1s constrained by the shorter, colder Alberta growing season. Geographic variation 1n adult size and sexual dimorphism was investigated in light of the differences in offspring development. Adult skull and wing variables indicate that this species conforms to Bergmann's rule in that Alberta bats are larger. While sexual dimorphism is not pronounced in New Hampshire, it is evident in Alberta bats. The most striking result 1s the larger wing size of Alberta females. Much of the variation in M. lucifugus adults can be explained by selection for larger offspring which affects female morphology. A new hypothesis incorporating the effects of selection during offspring development is advanced to explain patterns of geographic variation 1n size and sexual dimorphism among birds and mammals with broad latitudinal distributions.