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The reproductive biology of short mackerel, Rastrelliger brachysoma, which forms spawning aggregation off Pattani Bay was measured by gonadosomatic index (GSI), size at first maturity (L50) and morphometric maturity classification. We found an unbalanced sex ratio between males and females (1.60:1, p<0.0001). Two peaks of GSI were coincident with two spawning seasons; the pre-monsoon (February) and the south-west monsoon (May, June, August, and September). The L50 was estimated to be 17.8 cm and 18.3 cm (TL) for male and female, respectively. The morphometric classification indicated that 50% of R. brachysoma population off Pattani Bay matured at 16.2 cm (TL) and 14.7 cm (FL). Positive relationships were found between fecundity and total length, total weight, and ovary weight. The occurrence of immature and maturing stages found throughout the year indicated that the pattern of catch from this habitat might affect fish recruitment and stock.