The reproductive biology of the blue swimmer crab, Portunus pelagicus, was investigated in Wallis Lake, the largest fishery for the species in NSW. Crabs were sampled from fishery-dependent and -independent sources between November 2002 and July 2003. Mature female and male crabs were present in each month throughout study. Mature female (stage 5 gonads) were most prevalent in November, but there was no distinct pattern for males. The estimated size at which 50% of female and male P. pelagicus reached maturity was 46 and 44 mm CL, respectively. Fecundity of individual crabs was estimated to be between approximately 463, 000 and 1,781,000 eggs per brood. No significant difference in the CL - fecundity relationship was found for the two months examined (November 2002 and February 2003). The estimated number of egg broods produced (NBj) by female crabs in the different size classes ranged from one in crabs (50 - 55 mm CL) to three in crabs (75 - 80 mm CL). The reproductive biology of P. pelagicus is compared to reports from elsewhere throughout its distribution and implications for managing populations in NSW are discussed.