In the majority of sexual species, there are asymmetries in reproductive effort, with males typically investing more in securing matings and females investing more in producing offspring. This causes males to mate less discriminately than females. Yet males may also become choosy if the following conditions are met: (a) that females vary in their reproductive value, (b) that males can perceive this variation, and (c) that mating with one female reduces the possibility of mating with another. These conditions may be met in the livebearing Goodeidae, a clade of Mexican fish whose females are often brightly coloured and whose males display costly ornaments and courtship as the only means to obtain matings. Males of the black-finned goodeid (Girardinichthys viviparus) have relatively simple, yet costly courtship behaviour, with mating probability depending on the duration of one-to-one courtship episodes, thus by courting one female they must ignore others. We evaluated whether the decision to court a female depends on her phenotype. Three variables of female phenotype were positively linked to the duration of male visits and to the frequency of displays performed by males: belly area, hue (“orangeness”) and size. Since fecundity and offspring survival were also a positive function of female size, we conclude that male G. viviparus evaluate the potential female reproductive value and allocate their courtship effort accordingly. Since male courtship effort is also influenced by female colouration, we suggest that our findings may help explaining the recurrent evolution of sexually dimorphic female colouration in this clade.