Poeciliidae are a family of viviparous fishes characterized by reversed sexual size dimorphism. Although in all species males are smaller than females, there is a large variability in size dimorphism both between and within species. It has been suggested that variation in the degree of sexual size dimorphism across the family is related to differences in intensity and direction of both intra‐ and intersexual selection. Here the relationship between male body size and in‐trasexual competition, and its effect on male mating success, have been studied in four poeciliid species. Wide interspecific variation was found in the intensity of male‐male competition and in the importance of body size to achieve dominance. InBelonesox belizanusandGirardinus falcatus, larger dominant males almost completely excluded other males from access to the females. Males ofPhalloceros caudimaculatusare less aggressive and dominance influences their access to females to a lesser degree. InHeterandria formosa, aggressive interactions are frequent, but the outcome of fighting and the access to females is not influenced by the size of competitors.