Studies of seastar larvae in the Derwent River (Tasmania) were hampered by identification uncertainties. A genetic test was developed, based on PCR amplification of a 1300 bp mitochondrial DNA region followed by digestion with three restriction enzymes. The restriction profiles of adults of 14 seastar species were determined. The test was validated for larvae as laboratory-raised larvae of two species had the appropriate composite haplotypes. Approximately 80% of planktotrophic seastar larvae from Derwent River plankton samples were identified as the recently introduced northern Pacific seastar, Asterias amurensis. The two other larval seastars identified were Coscinasterias muricata and Patiriella regularis.