In this paper, we present the results of cellular and molecular studies on the neuroendocrine correlates of male sexual polymorphism in a population of the blenniid fish Salaria pavo (Risso). Bigger and older males defend nests, whereas smaller and younger males mimic female nuptial coloration and behavior to gain access to nests and sneak fertilizations. In this population, sex-role reversal in courtship also occurs (i.e., females are the courting sex). Immunocytochemistry and in situ hybridization were used to examine the production of arginine vasotocin (AVT) peptide and messenger RNA, respectively. The expression of AVT mRNA on a per-cell basis was correlated with mating behavior, rather than with sex morphotype, which suggests that the greater level of AVT mRNA expression in females and sneakers is correlated with the production of courtship behavior. On the other hand, the number and size of AVT peptide-producing cells in both male types is higher or larger, respectively, than in females, which suggests that it is correlated with sex morphotype, rather than with mating behavior.