We will produce DNA probes that identify unambiguously the genetic sex of European eel. Both species' genetic and phenotypic sex do not necessarely match; moreover sex inversions occur commonly, such probes will facilitate selection of suitable fish for aquaculture. Genotypic and phenotypic sex do not necessarily coincide in fish; very often sex inversions or environmentally induced sex differentiation are observed. As a consequence many aquacultured stocks show an unbalanced distribution of the sexes. If the genotypic sex would be known, a strategy could be more easily developed to influence sex differentation and to develop monosex populations. Our strategy to identify the genotypic sex in fish includes two major goals: (1) the development of DNA probes of repetitive non-coding sequences associated with differentiated sex chromosomes and (2) the development of DNA probes of sequences related to the sex determining gene. The fish species Leporinus, Poecilia (guppy) and Oncorhynchus (rainbow trout) will be used as reference species (they carry well studied gonosomes), while Dicentrarchus (sea bass) and Anguilla (European eel) are the target species. Such information will facilitate aquaculture by providing effective means to monitor the mechanisms which cause the switching of phenotypic sex. It is expected that at the end of the project various probes will be available to the aquaculture community which are suitable for identifying the genotypic sex of sea bass, European eel and possibly other commercial species. We will also procude probes based on the sex gene proper which could be diagnostic in other fish species.