abstract:Reproductive biology and seasonal condition cycle of the most dominant flatfish species Pseudorhombus pentophthalmus, in surrounding waters of Gui-Shan Island, northeastern Taiwan were examined based on the samples taken from March 2004 to March 2005. The spawning of R pentophthalmus took place between December and June, and peaked in February based on the seasonal changes in gonadosomatic index (GSI). The size at first maturity for females is 14.2 cm, but is 13.8 cm for males. Sex ratio of the species skewed to the male for most of the months, especially during the spawning season (P<0.05). Fishes less than 16 cm were dominated by males, while it was dominated by females after such a size. The hepatosomatic index (HSI) and visceral index (VI) of P. pentophthalmus both increased during the spawning season, when condition factor (CF) decreased. All indices dropped to the lowest value after spawning. The pattems indicated that P. pentophthalmus might reduce feeding activity and used reserved energy, mainly in the muscle, for spawning. P. pentophthalmus in northeastern Taiwan matured at a larger size, and exhibited an earlier and more protracted spawning season than those found off Japan. Geographical variations in reproductive biology suggest that P. pentophthalmus is able to adjust phenotypic response to environmental conditions that vary with latitude.