The breeding biology of a gobiid flash Acentrogobius viridipunctatus was studied over a 13- month period in the mangrove area of Karachi Coast (24⁰51´ 30´´- 24⁰51´ 30´´ N, 66⁰ 54´- 66⁰ 58´ E) for maturity, sex ratio, spawning frequency and fecundity . The gonads were classified into seven maturity stages. The breeding seasons were found to extend from February to May with the peaks in March-April, spawning once a year as substantiated by changes in gonosomatic indices (GSI), gonad maturity rate (GMR) and monthly frequency of maturity stages. An overall male to female ratio is 1.24:1.00. The sexual maturity occurs at a slightly smaller size at 98mm in females than in males at 110 mm TL. The absolute fecundity is 12447-36058 (2007±6952), the relative fecundity 905±227 and ova/g ovary weight 11699±5972. The mathematical equations describing the fecundity relationships with length and weight of the fish and ovary have been estimated. Chi-square test indicates that the fecundity- fish weight relationship is more valid than the fecundity-length relationship.