Ecological causes for the evolution of sexual dimorphism can be confidently inferred only when the sexes differ in morphology or relative size of the feeding apparatus, in a direction inconsistent with that expected from sexual selection. Snakes are well suited for such an analysis because head sizes in this group are important for feeding but not for reproductive behavior. My data reveal significant sexual dimorphism in head size (relative to snout-vent length) in 47% of 114 species examined from seven families. Head size relative to body size is strongly correlated between males and females in comparisons among populations within species, among species within genera, and among genera within families. Hence, correlation between the sexes may powerfully constrain evolutionary shifts in dimorphism. Nonetheless, phylogenetic analysis identifies many independent origins and losses of the dimorphism. Geographic variation in relative head size and sexual dimorphism in head size are evident within wide-ranging species. Dietary divergence between the sexes occurs in a taxonomically and ecologically diverse group of snakes. The ecological advantages of head-size dimorphism probably do not involve competitive displacement between the sexes. I infer that the sexes originally diverged in body sizes or ecology because of differences in reproductive biology, with the subsequent evolution of sexual dimorphism in feeding structures through independent adaptations to foraging in each sex.