In teleosts, estradiol-17β (E₂) is an important hormone responsible for oocyte development. To elucidate the molecular mechanisms underlying E₂ biosynthesis, we characterized the structure of red seabream (Pagrus major) cytochrome P450 aromatase (P450_arom) that is directly involved in E₂ biosynthesis and found changes in mRNA levels of P450_arom during oocyte development induced by implantation of gonadotropin-releasing hormone analogue. A cDNA clone encoding P450_arom is 1779 base pairs in length and encodes a protein of 519 amino acids in length, with a calculated molecular weight of 58.9 kDa. Northern blot analysis showed that P450_arom mRNA levels increased gradually from Day 8, when oocytes reached the secondary yolk globule stage, and were maintained at high levels at the day of spawning (Day 15). The P450_arom mRNA levels increased in association with an increase of the gonadosomatic index (gonad weight/body weight × 100%), serum E₂, and P450_arom enzyme activity (in vitro conversion of testosterone to E₂ in the ovarian fragments). Furthermore, an increase in mRNA levels of the LHβ, but not FSHβ, correlated with increased P450_arom mRNA levels during the course of ovarian development. In addition, the levels of P450_arom mRNA increased in isolated ovarian follicles during the course of vitellogenic oocyte growth and became undetectable in follicles at the migratory nucleus and the mature stages. These findings, together with those of the previous studies, suggest that LH, not FSH, may regulate E₂ biosynthesis via increased levels of P450_arom mRNA during oocyte development of red seabream.