Allozyme electrophoresis was used to identify the postlarvae of two species of tiger prawns, Penaeus esculentus and P. semisulcatus, from the Gulf of Carpentaria, Australia. The two species were examined for genetic differences at 39 loci, in three developmental stages (adult, juvenile and postlarval). One locus, Gpi, was suitable for the routine species identification of postlarvae. The electrophoretic identification system developed, when applied to the identification of postlarvae taken by field sampling, was more reliable and efficient than standard morphological techniques.