The gonadal structure and cycle of the Leguminaia whaetleyi is described for the first time indicating that 5% of this species is simultaneous hermaphrodite. A total of 420 individuals were collected in monthly samples from October 2008 to September 2009 in Gölbaşı Lake, Turkey. Calculation of the condition index and histological examination of the gonads showed that gametogenesis began in January. Spawning occurred between May and August with one maximum peak in July. Annual maximum oocytes size peaked in July. The population consisted of simultaneous hermaphrodite individuals. Sex ratio of L. whaetleyi was significantly different from the expected 1:1 ratio (P<0.05), and also female biased sex ratio was recorded. In hermaphrodite specimens, male and female follicles were mixed in the visceral mass but can be clearly distinguished by light microscopy study. L. whaetleyi appears to be a dioecious species in which 25 specimens were simultaneous hermaphrodite and under certain environmental conditions may be capable of self-fertilization. This suggests that detailed studies on sex ratio of L. whaetleyi may be helpful in understanding unionid phylogeny. J. Exp. Zool. 315:30–40, 2011. © 2010 Wiley-Liss, Inc.