Poeciliids, a family of live-bearing freshwater fish, including among others platyfish, swordtails and guppies, fully illustrate the diversity of genetic sex determination mechanisms observed in teleosts. Besides unisexuality, a variety of sex-determining systems has been described in this group of fish, including male and female heterogamety with or without autosomal influence, as well as more complicated situations such as multichromosomal and polyfactorial sex determination. Due to the presence of different mechanisms in closely related species or even between populations within a same species, poeciliids are a very attractive model to study the evolutionary dynamics of sex determination. For one species, the Southern platyfish <i>Xiphophorus maculatus, </i>positional cloning of the master sex-determining gene has been initiated through the construction and sequencing of bacterial artificial chromosome contigs covering the region differentiating the X from the Y chromosome. Initial analysis revealed a high plasticity of the sex-determining region and the absence of synteny with other fish and vertebrate sex chromosomes, indicating an independent evolutionary origin.