Steroids have been widely used in aquaculture for sex control and generally androgens drive testis differentiation while estrogens ovarian differentiation. However, even if these effects are well documented, the mechanism of action of these molecules still remains poorly understood. Past studies looking at a few candidate genes in all-male and all-female populations of rainbow trout, have confirmed the clear implication of steroid enzymes in gonadal differentiation in this species. We more recently switch to another approach by looking at gene expression of numerous putative mediators or targets of steroid action. These genes belong to the following families : transcription factors (SF1, WT-1, SOX9, DAX1, DMRT1, FOXL2,...), specific germ cells proteins ( Vasa, Germ cell less,...), steroidogenic enzymes (3betaHSD, P450scc, P450c11, P450aro,...), hormones and growth factors (TGFbetas, IGFs, AMH,...), apoptosis regulators ( Caspases, BCL2, Survivin,...) and hormone and growth factor receptors ( receptors for LH or FSH, androgens and estrogens, IGFs,...). The results of this study bring a lot of new interesting data on gene expression during rainbow trout gonadal differentiation with especially some gene clusters that clearly discriminate between testicular and ovarian differentiation. Among these clusters, two clusters represent early up-regulated gene expressions either during testicular ( AMH, P450c11, DMRT1,...) and ovarian (P450aro, FOXL2, LH receptor...) differentiation.