P>Age at length, growth and reproduction of 220 Mastacembelus mastacembelus specimens from the Ataturk Dam Lake were studied from July 2005 to July 2006. Total lengths and weights ranged from 7.0 to 85.0 cm and from 6 to 1100 g, respectively. Maximum age was 13 years. The regression model fitted to length and weight data was W = 0.0228 L-2.43 for males and W = 0.0029 L-2.95 for females. The von Bertalaffy growth equations for males and females were L-t = 99.2 [1-e(-0.11 (t-0.12))] and L-t = 69.2 [1-e(-0.26 (t-0.35))], respectively. Males dominated especially at an older age; the overall sex ratio was 1 : 0.63 (M:F). The breeding period was from May to July. Fecundity ranged from 2540 to 24 000 eggs per female.