– Population structure, growth, and reproductive characteristics of 689 Siahmahi, Capoeta damascina , a native species of Cyprinidae family in Zayandeh Rud River were investigated by monthly sampling during March 2007 to April 2008. Age groups of males and females were 1 + to 8 + and 1 + to 10 + , respectively. The sex ratio was 1:1.57 (male: female). The largest female was 54.2 cm in fork length and 2340 g in weight and the largest male was 39 cm in fork length and 1115 g in weight. The most frequent age groups were 3 + and 4 + in males and females, respectively. The highest value of condition factor was observed in June. The minimum, maximum, and absolute fecundity were 2523, 72645, and 24811±17833, respectively. Histological studies and gonadostomatic index (GSI) values suggested that spawning of C. damascina occurs from May to June. Thus, C. damascina is considered a synchronous species. Growth parameters for both sexes were stated by the von Bertalanffy growth function, as fallows: k = 0.1359, L ? = 62.61 cm, t o = -0.1031 years for females and k = 0.3001, L ? = 40.67 cm, t o = 0.468 for males. The weight-length relationship was described as W = 0.0155 L 2.9867 (r 2 = 0.99) for females and W = 0.0169 L 2.9469 (r 2 = 0.98) for males. The growth performance index (?`) was estimated as 6.31 and 6.21 for females and males, respectively, indicating a faster growth rate for females. Key words: Age, Cyprinidae, Fecundity, GSI, Iran, Histological, Reproduction