The proportion of berried (externally egg-bearing) females, fecundity, gonadosomatic index, and egg size of the blue swimmer crab Portunus pelagicus off southern Australia were examined by analysing data from samples of commercial catches using generalized linear models. Ovarian development was studied morphologically and histologically. Female blue swimmer crabs spawn from October to January and can spawn more than once per season. The fecundity of female blue swimmer crabs initially increased with carapace width, maximized at a carapace width of 134 mm, and decreased thereafter. Thus, fecundity increased 83.9% with an increase of carapace width from 105 to 125 mm, implying a single large female can produce as many eggs as two or more small females. The gonadosomatic index of female crabs from Spencer Gulf increased 2.4% from October 1998 to November 1998, increased an additional 15.9% by December 1998, and then decreased 62.3% to its minimum in January 1999. Gonadosomatic index also increased with 1/3 power of their body weight. Thus, the commonly used gonadosomatic index is overly simplistic for blue swimmer crabs. Unlike tropical or subtropical female blue swimmer crabs which often continually spawn, those off southern Australia spawn only in certain times of the year. The implications of this work lead to suggestions for three management measures for the commercial fishery: maintaining the prohibition of catching berried females, extending the seasonal closure from October to January, and maintaining the current legal minimum carapace width of 110 mm.