We isolated the structural gene encoding cytochrome P-450 aromatase (P-450arom), for the first time from a nonmammalian vertebrate, the medaka (a teleost fish, Oryzias latipes), using the rainbow trout P-450arom cDNA as a probe. The structure of the entire P-450arom gene, the nucleotide sequence of its 5' flanking region, and the transcriptional initiation sites were determined. The medaka P-450arom gene consists of nine exons, but spans only 2.6 kb, being much smaller than the human P-450arom gene (at least 70 kb), as a result of extremely small introns (medaka, 73-213 by vs. human, 1.3-10 kbp). The splicing junctions are located at exactly the same positions as those found in the human P-450arom. The deduced amino acid sequence is 51-52% identical to those of mammals and chicken, and 75% identical to the rainbow trout amino acid sequence. Genomic Southern blots revealed the presence of a single medaka gene. Promoter analyses indicated two major transcription initiation sites 60 and 61 bp upstream from a putative initiation codon. The promoter region of medaka P-450arom gene also contains potential Ad4BP sites and estrogen responsive element (ERE) half-sites. These results suggest that the basic structural organization of P-450arom genes and the regulatory mechanisms of expression are well conserved through-out the vertebrates.