Abstract: Aspects of the reproductive biology of Hilsa shad, Tenualosa ilisha from Shatt al Arab river and Southern Al Hammar Marsh during spawning migration have been investigated from the period of April 2013 to March 2014. A total of 1456 specimens were collected using gillnets and landings. The size range of the specimens collected throughout the migration season was 50-460 mm in Total Length (TL). Males were dominant in the smaller lengths, while females did in the larger lengths. The monthly variation in mean length for both sexes showed two distinct peaks, the first was during February-March and the second during September-November. The overall sex ratio showed that females were dominating (1:1.7). The sex ratio varied both spatially and seasonally. Monthly variations in Gonad Somatic Index (GSI) and Dobriyal index indicated that Hilsa shad had a long spawning season with two distinct peaks in April and November. Macroscopic observations of gonads showed six maturity stages. The spatial occurrence of maturity stages suggested that Hilsa shad spawn in all the stations of Shatt al Arab river. The size at 50% maturity was 210 mm for females and 170 for males. The absolute fecundity ranged from 223750-1477780 eggs for fishes with total length range of 200-450 mm and mean weight range of 246.3-891.7 g. The relative fecundity for the same length and weight group ranged from 900.5-1657.1 egg/g. The relationship between absolute fecundity (AF) and total length was AF = 0.007×TL3.129 (R2 = 0.97, n = 62) and that between AF and body weight (Bw) was, AF = 190×Bw1.327 (R2 = 0.95, n = 62).