Differences in behaviour, such as the mate-locating tactic, may favour the evolution of interspecific and intersexual differences in the structure of the visual system. This notion was tested by examining the relationships between eye size, body size, sex and mate-locating tactic in 16 species of butterflies. The 16 species were grouped in eight phylogenetically close pairs that differed in the mate-locating tactic displayed by the males. Eye size was characterized by eye surface area, which was estimated from a set of linear eye measurements made on 10 individuals of each sex in each species. The major findings were that eye surface area was positively correlated with body size both among and within species, males have larger eyes relative to their body size than females, and after controlling for body size and phylogeny, interspecific differences in the mate-locating tactic of males does not explain interspecific variation in the size of male eyes or in the magnitude of sexual difference in eye size. The optical and behavioural consequences of variation in eye size with body size and sex will await a better understanding of how eye structure varies with size within and between species.