A scientific observer-based survey was used to quantify the retained and discarded catches in the largest trap fishery for the Portunid crab, Portunus pelagicus, in south-eastern Australia (i.e. Wallis Lake) during the 2002/03 fishing season. Overall 81% of male and 79% of female P. pelagicus captured were larger than the current minimum legal carapace length (i.e. 60 mm) and therefore retained. It was estimated that approximately 385,988 (± 37,551) P. pelagicus were retained and 183,653 (± 29,716) discarded from the trap fishery during the 2002/03 season. The quantities of non-target species captured were generally very low, and significantly less than that generally reported for other crab-trap fisheries. Acanthopagrus australis was the most abundant bycatch species, with an estimated total of 2,445 (± 1,564) caught, representing 43% of the total bycatch, during the 2002/03 season. Discarded organisms were in good condition when returned to the wild. This study demonstrates that the Wallis Lake trap fishery for P. pelagicus is relatively selective and has minimal direct impact on non-target species.