Population structure, growth, and reproductive characteristics of Mossul bleak, Alburnus mossulensis a, a native species of Cyprinidae family in Bibi Sayyedan River of Semirom, Isfahan were investigated by monthly sampling through December 2010 to December 2011. For this purpose, 543 speciments (253: famales, 159: males and 31: immature) were caught by sein net (5×5 mm mesh size) and a cast net. The collected sampeles were traorted to the laboratory on ice and subjected to dissection and biometric measurments. The recorded water temperature during the 13-month study period varied from 10.5 ?C to 21.4 ?C. Age groups of males and females were 0 + to 5 + . The sex ratio was 1:2.2 (male: female); chi-square test showed a significant difference in the sex ratio (p 0.01). Although the number of males was more than the number of females in younger age 0.0169 L 3.035 (r 2 = 0.954) for males, W = 0.0061 L 3.175 (r 2 = 0.975) for females and W= 0.0066 L 3.139 (r 2 = 0.977) for all fish. Regression coefficient (b) in males, females and all fish was higher than 3 and based on Pauly test, only in males it was not significantly different from b= 3 (p 0.05), indicating a positive isometric growth pattern for males and a positive allometric growth pattern for females and all fish. The most frequent age group was 2 + in both sexes. The age and length total at first maturity of females and males were estimated to be 1 years and 5.3-9.9 cm total length, and 2 years and 4.4-9.1 cm, respectively. The highest value of Gonadosomatic Index (GSI) was observed in March and May in males and females, respectively. Values of GSI during different months showed a significant difference in both sexes (p 0.05), but values of Hepatosomatic Index (HSI) during different months was not significantly different in both sexes (p 0.05). The fluctuations in GSI and HIS values were in congruent. Fecundity was measured in females captured prior to the reproductive period (March to May) and preserved in Gilsons fluid. The minimum, maximum, and avarge absolute fecundity were 2064, 10316, and 5505±2686 (±SD), respectively. The mean egg diameter was 0.62±0.15 (±SD) mm; the minimum and maxim um egg diameter was 0.1 and 2 mm. According to the results of macroscopic analysis of gonads and gonadostomatic index (GSI) values, 5 sexual maturity stage inculuding: immature, maturing virgin or recovered spent, ripening, ripe, a spent were observed. In general and according to the results of GSI, egg diameter and sexual maturity stages, spawning of A. mossulensis occurs from March to July. Thus, A. mosullensis is considered a multiple batch group synchronous species. Keywords : Cyprinidae, Fecundity , Age, Reproduction, Gonadosomatic Index, Allometric
