A full-length cytochrome P 150 cDNA, ("YP2K4. was assembled from mi deotide sequences obtained from 5'-RA('E, 3'-RACE and a clone (kidB) from screening of a /omMagtll rDNA library from the trunk kidney (TK) of postspawning (3y) male rainbow trout using a CYP2K1 full-length cDNA as a probe. The cDNA was 2601 bp in length and contained an open reading frame encoding for a protein of 504 amino acids and a calculated MW of 56,745 Da. The deduced N terminal amino acid sequence of CYP2K4 matches closely that of KM2, a reported male-specific P450 purified from mature male rainbow trout TK. Northern blot analyses were performed using the 3'-RACE PCR product f 1018 bp ,T-untranslated region) as a probe for the liver and kidney of 6-rnonth and 3y trout and 12 other tissues from 2y trout of both sexes. Only a 2.8 kb band was detected in the 3y male TK and liver and the TK had much higher intensity than liver. This specific transcript was not observed in any tissueexamined in 2y trout and the liver and kidney of 6-rnonth fish.