In contrast to the situation observed in mammals and birds, a switching between different sex determination systems frequently occurred during the evolution of the teleost fish lineage. This might be due to a frequent turnover of sex-determining signals at the top of the sex determination cascade (master sex-determining genes in the case of genetic sex determination). Alternatively, different variations of a same master gene might decide the sex of individuals in different sex determination systems. In the medaka Oryzias latipes, a Y-specific copy of the putative transcription factor gene dmrt1 very likely corresponds to the master sex-determining gene inducing male formation [Nature 417 (2002) 559; Proc. Natl. Acad. Sci. U. S. A. 99 (2002) 11778]. This gene, alternately called dmY and dmrt1Y, has been formed by duplication of the autosomal dmrt1. In order to determine if an orthologue of dmrt1Y was also located in the sex determination region of the related platyfish Xiphophorus maculatus, Southern blot analysis was performed on genomic DNA from XY and YY males and WY and XX females using both medaka and human dmrt1 cDNA as probes. Using different restriction enzymes, no evidence for sex-specific dmrt-containing genomic fragments could be found in the platyfish. No dmrt gene could be detected by low-stringency Southern blot analysis of genomic inserts from 60 bacterial artificial chromosome (BAC) clones linked to the sex-determining locus on the X and Y chromosomes. Six different groups of BAC clones containing dmrt genes were isolated from a platyfish genomic library. X. maculatus dmrt1 gene and cDNA were characterised. In adult, expression of dmrt1 was detected only in testis. Phylogenetic analysis indicated that the duplication of dmrt1 that led to the formation of dmY/dmrt1Y within the medaka lineage clearly occurred after its divergence from the platyfish lineage. Hence, a dmY/dmrt1Y orthologue generated by this particular event of duplication does not correspond to the master sex-determining gene in the platyfish.