A 548 bp partial cDNA fragment of 17-hydroxysteroid dehydrogenase (17-HSD1) was obtained by RT-PCR from the ovary of Nile Tilapia. The expression of 17-HSD1 was high from 0 to 11 days after spawning, but there was a sharp decline at the day of spawning (day 14) indicating its involvement in ovarian cycle.