Bergmann's rule predicts an inverse relationship between body size and environmental temperature in endotherms. The larger the organism, the lower the rate of heat loss by radiation; thus, larger organisms should be more likely to survive in colder places. This relationship is less clear in ectotherms. We tested Bergmann's hypothesis for a sample of 260 individuals of Anolis nebulosus from four Mexican populations ranging from a continental island to a forest at 2,100 m in elevation. Using analysis of variance, we tested for differences in body size (first principal component extracted from seven morphometric variables) among populations. Average body size was expected to be significantly larger in higher localities. Results did not conform to expectation; the island population was significantly larger in body size than the rest likely because of lower predation pressure. Also, we examined the relationship between body size, perch temperature (PT), and the difference between body temperature and PT. There were significant differences among populations in the range of PTs used. Individuals from highlands, where temperatures are colder and vary greatly, used a broader range of PTs and used perches that were colder than their body temperature. Only the largest individuals, mostly males, were able to use the coldest perches. In contrast, at lower sites, which had less thermal variation, lizards showed less variance in PTs, and there was no relationship between PT and size. In general, variation in PT and body size was smaller for females than for males. Results suggest a combination of heliothermy and heat conservation by size as regulatory strategies, and a trade-off between mating strategies and heat loss. For these populations, body size is one of a number of factors involved in temperature regulation; the particular combination of these factors on a population is probably a response to local environmental conditions.