The European shore crab Carcinus maenas is a highly successful marine invader, and has displayed rapid range expansion following its introduction to many parts of the world. In Australia, it was first reported in Port Phillip Bay, Victoria in the late 1800s. Despite predictions that it would expand its range northward, its distribution has remained limited to the southeast coast of the mainland and to Tasmania. Using microsatellite loci and mitochondrial DNA, we assessed whether low connectivity among southeastern Australian estuaries might be contributing to the limited distribution of this invasive species. Under this hypothesis, we expected that sampling of C. maenas from 6 estuaries, roughly evenly spaced along the southeast coast, would reveal: (1) greater genetic variability among than within estuaries; (2) increasing genetic dissimilarity with distance from Port Phillip Bay; and (3) in the absence of human-mediated dispersal, declining genetic variation with distance from Port Phillip Bay. Contrary to these predictions, we found that genetic variability was no greater among than within mainland southeast Australian estuaries-indicating significant gene flow. Some slight genetic differentiation was, however, evident between Tasmania and the mainland. Multiple introductions appear to have contributed to the Australian population. The high connectivity of populations among southeast Australian estuaries suggest that management strategies focused on eradication of C. maenas from individual localities will not be effective. Factors other than limited connectivity appear to be responsible for the slow range expansion up the east Australian coast.