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 (T_set, 20.0–26.5 C) measured in a laboratory thermal gradient were lower than those reported for most snake species. Physical models demonstrated that T_set 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 T_set availability) allowed snakes to achieve field body temperatures (T_b) 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 T_b than nongravid females, demonstrating a functional link between habitat use and thermoregulation. Differences in T_b between gravid and nongravid females were greatest at low ambient temperatures, highlighting their different thermoregulatory strategies. Maintenance of low T_b in nongravid females is consistent with an energy conservation strategy benefiting reproductive investment.