Levels of urinary progesterone and estradiol-17β were measured twice a week in a female dugong, Dugong dugon, in captivity for two years from April 1996 to April 1998. The dugong showed 14 ovarian cycles during the period of study. Concentrations of progesterone ranged from 0.01ng/mg creati-nine (Cr) to 1.94ng/mg Cr and the length of estrous cycle was 53.6±8.6 (mean±SEM) days based on intervals of urinary progesterone peak-to-peak measurements. Concentrations of urinary estradiol-17β ranged from 0.9pg/mgCr to 23.7pg/mgCr, and tended to peak just prior to elevations of progesterone during the first year of study. This is the first report demonstrates that the ovulatory cycle of the dugong is about 50 days. The present findings suggest that measurement of urinary progesterone is a useful method to detect ovarian cycle of the dugong in captivity.