Females of some temperate-zone snake species appear to exhibit thermophilic behavior during gestation, resulting in differential habitat use between gravid and nongravid individuals. We investigated thermoregulation of 13 female cottonmouths (Agkistrodon piscivorus) near their northwestern range limit in southwestern Missouri during mid-summer. Mean preferred body temperature (23.4 C) and preferred body temperature range (Tset, 20.0–26.5 C) measured in a laboratory thermal gradient were lower than those reported for most snake species. Physical models demonstrated that Tset consistently was available to snakes during the study period, although the frequency of availability varied among habitats. The relatively high thermal quality of the environment (based on Tset availability) allowed snakes to achieve field body temperatures (Tb) closely matching laboratory preferences. Gravid females exploited the thermal environment more efficiently than nongravid females by preferentially occupying the most thermally favorable microhabitats. Accordingly, gravid females consistently maintained higher Tb than nongravid females, demonstrating a functional link between habitat use and thermoregulation. Differences in Tb between gravid and nongravid females were greatest at low ambient temperatures, highlighting their different thermoregulatory strategies. Maintenance of low Tb in nongravid females is consistent with an energy conservation strategy benefiting reproductive investment.