P450scc and 3 beta-HSD cDNA were isolated from a zebrafish lambda gt10 cDNA library using trout SCC and 3 beta-HSD cDNA as the probes. The zebrafish SCC cDNA encodes a protein of 509 amino acids, which shares a 78% similarities with the trout SCC and 58% with the human SCC. As for 3 beta-HSD, two forms of cDNA were isolated, termed HSD 5 and HSD 17, which may have resulted from alternative splicing. HSD 5 and HSD 17 encode proteins of 374 and 341 amino acids respectively. Both share 77% amino acid similarities with trout 3 beta-HSD and 53% similarities with the mouse 3 beta-HSD. Zebrafish has been increasingly used as a genetic model system to study organ development and to investigate human diseases. The cloning and the characterization of zebrafish P450scc and 3 beta-HSD should facilitate study of steroidogenesis and human disease associated with steroid imbalance.