Some aspects of reproductive biology of the king nase, Chondrostoma regium, a cyprinid fish, were investigated. Some 333 specimens of female king nase were collected in Bibi-Sayyedan River from March 2010 to March 2011. The gonadosomatic index and egg diameter were measured. The gonadosomatic index showed a gradual increase from May to March with a peak in March. Eggs were released at once. Based on the high GSI index in March and sharp decline during the next few months, it can be concluded that this species has a short spawning period in spring. The egg diameter ranged between 0.267 and 1.70 mm with a mean of 0.65±0.01mm (±SE). These variations are consistent with the variation in the gonadosomatic index.
