Portunus pelagicus (blue swimmer crab) is widely distributed and fished throughout the indo-west Pacific, but little is known of its biology and ecology in southeast Australia. In this study, the reproductive biology of P. pelagicus inhabiting Wallis Lake, a large coastal lake that supports the largest regional commercial fishery for the species was investigated. Males and females with mature gonads occur throughout the fishing season (November to July), however, ovigerous females are most prevalent in November and December. The mean size at which 50% of females reached maturity is estimated at 46 mm carapace length (CL). Fecundity increased linearly with CL and females are capable of producing up to 3 broods of eggs within the observed spawning season, with crabs of 60-69 mm CL producing approximately 76% of the estimated total egg production. Increasing the current minimum carapace length above 60 mm CL would potentially provide greater protection to mature females and increase total egg production, but this would reduce total retained catches in the fisheries.