Seven species of. sphaeromatid isopod colonize the shallow littoral zone of the Swan River Estuary. Their distribution relates closely to the hydromorphology of the estuary and even reflects small scale interannual changes in salinity. The high diversity can partially be explained by the heterogeneous environment and the establishment of introduced species. Possible sources of the sphaeromatid fauna are discussed using the criteria of a test proposed by Chapman & Carlton (1994) to identify introductions.