To characterize the involvement of the aromatase gene during the process of sex determination in the European eel (Anguilla anguilla), the expression of its gonadal form was determined during various developmental stages. The cloned cDNA from the European eel gonad (EeCYP19) contains an open reading frame of 1539 bp, encoding a deduced protein of 513 residues. The predicted amino acid sequence shows 97% identity with that of the Japanese eel, and 59-69% of identity with those of the CYP19 gonadal and brain forms of other teleost fish. Two potential initiation sites (ATG) were found downstream of the first ATG codon. A fluorescent-based method of real-time PCR was developed to quantify EeCYP19 expression. The expression levels of EeCYP19 in the gonads of adult males were approximately 12- and 30-fold lower than the levels in adult females and juvenile eels previously treated with E2, respectively. Expression of aromatase was found only in a single specimen in the control group. In contrast, no difference was found among sexes in the aromatase expression in the brain. Treatment with aromatase inhibitor (AI) of juvenile eel resulted in the total loss of aromatase expression in the gonads and brains. The results of this work revealed that AI treatment not only reduces the synthesis of estradiol, but reduces the expression levels of EeCYP19 as well. No evidence for the presence of a distinct extra-gonadal (brain) form of aromatase in the European eel could be provided.