In order to better understand the molecular and evolutionary mechanisms driving sex determination in fish, bacterial artificial chromosome (BAC) contigs covering the sex determination region of the X and Y sex chromosomes of the platyfish Xiphophorus maculatus have been constructed. Initial analysis of these contigs led to the identification of eleven gene candidates closely linked to the master sex-determining locus and expressed in different types of tissues. These genes are all present on both the X and the Y chromosomes, suggesting a restricted degree of differentiation and probably a young evolutionary age for the platyfish sex chromosomes. Syntenies were detected with several zebrafish chromosomes as well as with two medaka and several tetrapod autosomes, supporting an independent origin of sex chromosomes not only in different vertebrate lineages but also in different fish species.