Alburnus istanbulensis (Battalgil, 1941) is a commercially valuable endemic species which exists in the coastal streams of Marmara Region's northern sectors. 317 individuals, which have been collected from Darlik Dam, were examined during December 2011- December 2012. The highest GSI values were 13.07 +/- 1.63 for females and 6.95 +/- 1.28 for males in the beginning of June. The lowest GSI values were observed in August for females (1 +/- 0.33) and in September for males (1.05 +/- 0.21). By comparing the histological findings with macroscopic observations and GSI values, five developmental stages were determined for females, four developmental stages for males. These findings revealed that A. istanbulensis spawn in June.