The role of ecological factors and phylogenetic constraints in determining the mating system were examined for three species of chaenopsid blennies from the Gulf of California, Mexico. The mating system does not vary from resource defense polygyny, because of two phylogenetic constraints. First, the presence of demersal eggs and male defense of a spawning site preadapts males to parental care. Second, the microhabitat of vacant invertebrate tests, which serve as refuges and egg deposition sites, prohibits alternative male reproductive strategies, and ensures a role for epigamic selection. Factors determining the reproductive success of male were examined for three species. Females of Coralliozetus angelica, Acanthemblemaria crockeri, and Emblemaria hypacanthus exhibited a preference for large males in laboratory mate-choice experiments, which was reflected in the field where male body size was positively correlated with male reproductive success. The elevated dorsal fin of male E. hypacanthus was shown to be positively allometric, and may have evolved as a signal of body size. Both sexes of E. hypacanthus exhibited risk averse behaviors in an area of high predator density. Females of A. crockeri and C. angelica avoided spawning with males defending heavily fouled shelters. Some of the largest males of C. angelica may realize no reproductive success, because they are forced to defend heavily fouled shelters. The intensities of male courtship coloration and displays were negatively correlated with male reproductive success in A. crockeri and may have resulted from female preference for mated males. Female mate choice appears to be based on factors that are reliably correlated with the quality of male parental care. A number of factors impact the intensity of sexual selection on males. First, intrasexual selection is important, since field experiments demonstrated that shelter availability limits local population densities of male C. angelica, and shelter quality is a criterion of female choice. Second, epigamic selection plays an important role, because males are typically unable to usurp multiple shelters. Third, sex ratios are frequently male-biased, because males defending shelters gain an indirect survival benefit. Finally, egg-laying surface area may be limited, restricting female preferences.