A global analysis of gene expression has been carried out using trout cDNA microarrays to identify new paracrine factors involved in the onset of spermatogenesis in trout. Data showed that two ESTs were negatively regulated as spermatogenesis progresses and expression was localized to the Sertoli cells. The tissue distribution was limited to the gonad, particularly in the male, and a differential expression was observed between male and female during the sex differentiation period. The phylogenetic analysis of the two ESTs, termed GSDF1 and GSDF2, indicates that the encoded proteins belong to new subclass of the TGFbeta super family, GSDF1 being conserved in other Euteleteostean fish.