A Sox (SRY-type HMG box) gene, designated SoxP1, was isolated from a cDNA library made from pituitaries of immature rainbow trout. Sequence analysis indicated that the cDNA had an open reading frame encoding 467 amino acid residues containing a DNA binding motif, known as the high mobility group (HMG) box. Northern blot analysis showed trout SoxP1 mRNA was detected in pituitaries and gonadal tissues, but not in liver, spleen, and heart. In pituitaries, trout SoxP1 mRNA was more abundant in immature fish than in mature fish. Gel shift retardation analysis indicated that the recombinant HMG box protein of SoxP1 produced in E. coli had a DNA binding property for an AACAAT or AACAAAG sequence. These findings suggest that the trout SoxP1 protein may play certain roles in growth or maturation in pituitaries as a transcription factor.