The Australasian burrowing isopod (Sphaeroma quoianum) has been introduced to numerous embayments along the Pacific coast of North America. In some bays, populations of S. quoianum can exceed tens of thousands of individuals m-3 and bioturbation by the isopods can exacerbate shoreline erosion. Within their native range, however, studies recognize S. quoianum primarily as a woodborer. We measured the distribution, prevalence, habitat use, density, and associated fauna of S. quoianum in two bays within the native range [Tamar estuary (Tamar), Tasmania and Port Phillip Bay (PPB), VIC, Australia] and in one bay where the isopods had been introduced (Coos Bay, OR, USA). Distribution, prevalence, and habitat use were determined from shoreline surveys. Densities and the associated fauna of S. quoianum were measured in three intertidal substrata (marsh bank, wood, and friable rock). In all embayments, S. quoianum occurred primarily between 5 and 30 salinity and 55–68% of sites harbored isopods. Habitat use varied between embayments. Distributional patterns suggest salinity is the primary factor that limits the establishment and spread of S. quoianum. Isopod densities in all substrata were greater in Coos Bay than in the Tamar or PPB, although only densities within marsh banks varied significantly. Similarities in the amount of habitat and food, and the burrow dwelling lifestyle of S. quoianum suggest habitat availability/quality, food levels, predation, and competition are not responsible for the large differences in density. Lack of parasites or disease in populations of S. quoianum introduced to Coos Bay could be responsible for the prolific densities observed.