Sexual dimorphism of phenotypic traits associated with resource use is common in animals, and may result from niche divergence between sexes. Snakes have become widely used in studies of the ecological basis of sexual dimorphism because they are gape-limited predators and their head morphology is likely to be a direct indicator of the size and shape of prey consumed. We examined sexual dimorphism of body size and head morphology, and sexual differences in diet in a population of Mexican lance-headed rattlesnakes, Crotalus polystictus, from the State of México, Mexico. Maximum snout–vent length of males was greater than that of females by 21%. Males had relatively larger heads, and differed from females in head shape after removing effects of head size. In addition, male rattlesnakes showed positive allometry in head shape: head width was amplified while snout length was truncated with increased head size. In contrast, our data did not provide clear evidence of allometry in head shape of females. Adults of both males and females ate predominately mice and voles; however, males also consumed a greater proportion of larger mammalian species, and fewer small prey species. The differences in diet correspond with dimorphism in head morphology, and provide evidence of intersexual niche divergence in our study population. However, because the sexes overlapped greatly in diet, we hypothesize that diet and head dimorphisms in C. polystictus are likely related to different selection pressures in each sex arising from preexisting body size differences rather than from character displacement for reducing intersexual competition.