We report the cloning of a new trout myogenic cDNA which encodes helix-loop-helix protein homologous to the myogenic factor myogenin. Northern analyses indicate that trout myogenin (Tmyogenin) transcripts accumulate in large amounts in the myotomal musculature of embryos and frys. In adults, transcripts concentrate within the thin lateral layer of red (slow oxydative) muscle fibres. They are present only in low amounts in white (fast glycolytic) muscle fibres which constitute the major part of the trunk musculature. Using an in vitro myogenesis system, we observed that the trout myogenin encoding gene is not activated until myosatellite cells fuse to generate multinucleated myotubes, indicating that Tmyogenin lies downstream of muscle determination factors. All these observations show that in a major taxinomic group like teleosts, a gene with homology to myogenin exists. Its activation during myogenesis suggests that it acts as a major developmental regulator of muscle differentiation.