Males in many animals use alternative mating tactics to parasitize the mating effort of large dominant males. The primary cost of ‘peripheral’ males has been assumed to lie in stolen fertilizations and thereby reduced paternity of the dominant males whose mating effort is parasitized. Here, it is shown that for two nesting labrid fishes, Symphodus tinca and S. ocellatus, the loss of mating opportunities due to the presence of peripheral males exceeds the cost of paternity reduction by at least three and a half times. Experimentally reduced numbers of peripheral males in the wild revealed that when the majority of these were removed from nests, females of both species increased their spawning rate five-to eight-fold within minutes. Involvement in matings by peripheral males entailed no significant cost to females in fertilization rate, egg mortality or in the quality of subsequent parental care. The only hypothesis for this strong discrimination against peripheral males that cannot presently be refuted is that females choose mates based on age, defensive ability or size as an indicator of their genetic quality.