Although the sex-determining genes are known in mammals, Drosophila, and C. elegans, little is known in other animals. Fishes are an attractive group of organisms for studying the evolution of sex determination because they show an amazing variety of mechanisms, ranging from environmental sex determination and different forms of hermaphroditism to classical sex chromosomal XX/XY or WZ/ZZ systems and modifications thereof. In the fish medaka, dmrt1b(Y) has recently been found to be the candidate male sex-determining gene. It is a duplicate of the autosomal dmrt1a gene, a gene acting in the sex determination/differentiation cascade of flies, worms, and mammals. Because in birds dmrt1 is located on the Z-chromosome, both findings led to the suggestion that dmrt1b(Y) is a "non-mammalian Sry" with an even more widespread distribution. However, although Sry was found to be the male sex-determining gene in the mouse and some other mammalian species, in some it is absent and has obviously been replaced by other genes that now fulfil the same function. We have asked if the same might be true of the dmrt1b(Y) gene. We find that the gene duplication generating dmrt1b(Y) occurred recently during the evolution of the genus Oryzias. The gene is absent from all other fish species studied. Therefore, it may not be the male-sex determining gene in all fishes.