Snakes, as ectotherms, rely heavily on environmental temperature to drive physiological processes. Activity in terrestrial snakes is influenced heavily by environmental temperature, but climatic, temporal, and ecological factors play a role as well. Understanding when and under what conditions a species is most likely to be active on the surface can be important when dealing with a cryptic species and a species that occurs at low densities. Surface activity, body temperature (T_b), and microclimatic data were collected during a study of 17 Heterodon platirhinos in an early-successional sand-dune ecosystem from 2009 to 2011 using temperature-sensitive radiotransmitters. Mean T_b (27.9°C, SE = 0.24) and T_b range (5.5–39.5°C) were similar to those reported in other studies of H. platirhinos. Females were generally more active than males, except in September when males may be searching for mates. Mixed-effects logistic regression was used to assess the relationship between snake surface activity and selected microclimatic and temporal variables. Hours postsunrise and relative humidity emerged as the top variables influencing surface activity in gravid females. Hours postsunrise and ambient air temperature emerged as the top variables influencing surface activity in males. Our data suggest that activity in H. platirhinos occurs as a function of several environmental, temporal, and ecological factors and varies on the basis of sex and reproductive class. When considering activity, future studies on terrestrial ectotherms should consider not only the relationships of sex, but of reproductive class as well.