Within a pangenomic screen using rainbow trout cDNA microarrays we identified one clone with a clear testicular specific expression during gonadal differentiation. This microarray profile was confirmed by real-time quantitative PCR, and homology searches reveal this gene encodes a T-box transcription factor exhibiting a good homology with other vertebrates T-Box 1 genes. This gene, tbx1a, is widely expressed in different rainbow trout tissue including the adult testis and ovary. This is the first report in any vertebrate of the implication of a T-box 1 gene during testicular differentiation.