Although temperature is recognized as an important barrier to reptile distribution, the mechanisms of this limitation are poorly understood. We investigated the association between temperature, habitat selection, and thermoregulation in a population of massasauga rattlesnakes (Sistrurus catenatus) near their northern range limit, where low temperatures are likely an important constraint. Movement data indicated that the lower limit of the massasauga's performance range (i.e., ≥ 50% of optimal) was 19.9 °C. Operative environmental temperatures were almost always below the massasauga's preferred range (30.0–33.6 °C) and frequently below its performance range in spring and fall. Massasaugas differed from other northern snakes by thermoregulating primarily by microhabitat selection and not by macrohabitat selection. Massasaugas thermoregulated more effectively as their visibility increased and forest cover decreased. Gravid females increased thermoregulatory behaviour in response to low temperatures in spring and summer. Male and nongravid female massasaugas did not take full advantage of thermoregulatory opportunities in the spring and fall, and as a consequence were often too cool to perform essential functions such as prey capture. Our results are consistent with the hypothesis that temperature limits massasauga distribution primarily via effects on gestation and hence juvenile recruitment.