Huddling behavior in grouped house mice (Mus musculus) was observed at two ambient temperatures, and in groups containing one or both sexes. Increased huddling was observed in all sex combinations at the lower ambient temperature. The differences observed between groups varied with temperature: in general, male mice were more likely to alter their behavior in response to temperature and social situation, while the behavior of females was more consistent. The results are discussed in relation to the competing effects of thermoregulatory demand and Darwinian fitness.