The genetic and cell biological mechanisms making the decision whether the undifferentiated gonad of the embryo develops either towards male or female are manifold and quite different. Sex determining (SD) mechanism range from environmental to simple or complex genetic mechanisms and have evolved obviously repeatedly and independently. In species with genetic monofactorial sex determination, master SD genes lying on sex chromosomes drive the gonadal differentiation process by switching on a developmental program that ultimately leads to testicular or ovarian differentiation. So far very few sex determining genes have been identified in fish and in animals in general. Fish are uniquely suited to study the evolution of sex determination and SD genes. Comprising about half of the about 60 000 species of vertebrates, fish show also the greatest variety of sex determination mechanism including species with either environmental or genetic sex-determination. Even those few known SD genes are apparently not conserved over a larger number of related orders, families, genera or even species. This frequent evolutionary turnover of SD genes may be explained by the large diversity of these master sex regulators due to the high turnover of fish sex chromosomes. This project has then two major objectives: (1) to screen for potential sex determining genes in many fish species; (2) to demonstrate the role of identified genes as master sex determinants in a few selected species.