We report the cloning, tissue expression, and structural analysis of the aromatase gene in the rice field eel (Monopterus albus). The ovary-derived cDNA (1,802 bp) has a 49 bp 5'-untranslated region (UTR), a 202 bp 3'-UTR, and a 1,551 bp open-reading frame, which encodes a protein of 517 amino acid residues with a predicted molecular weight of 58.2 kDa. The amino acid sequence alignment suggests that the rice field eel ovarian P-450 aromatase shares 63-80% identity with that of other fish species, reduced to 59-60% with brain-derived aromatases of other fishes and to 50% with human placenta aromatases. Between the 5' and 3' untranslated terminal regions, the rice field eel CYP19 gene contained seven introns at the same sites as in medaka and human but lacked an intron between the I-helix and the aromatase-specific conserved region. All introns conformed to the GT/AG rule. Sequence analysis of the 1,065 bp upstream of the translation start site revealed that the transcription initiation site was 51 bp upstream from the translation start site. This region had one estrogen receptor recognition half site (nt -62), five copies of an SRY/iSRY binding motif, a C/EBP (CCAAT enhancer binding protein) binding site (nt -751), chicken ovalbumin upstream promoter-transcription factor (nt -986) and GATA-2 (nt -186, -249) recognition sequences, but no binding sequence for steroidogenic factor-1 and the cAMP response element binding protein activating transcription factor family. In females, levels of relative expression were, in descending order, hypothalamus, pituitary, forebrain, ovary, and liver. In males, P450arom was detected only in the pituitary and the liver, with half the expression found in females. In fry, the P450arom expression level increased during development and was significantly higher in the brain than in the gonad.