Only 5, possibly 6, of the 47 species of Plecoptera known to occur on Tasmania and the 42 on the Australian mainland are common to the two areas. Three of these occur on King Island at the western end of Bass Strait, but only one on Flinders Island in the east where the strait is shallowest. Other species on the two islands are Tasmanian. The reasons for these distributions are discussed. It is suggested that the endemism can be explained by inhospitable conditions in the strait area during periods of lowered sea level in the Pleistocene, and that the western route is probably the one by which a few species did move. It seems to have been a difficult route, and one stonefly species became isolated on King Island for long enough to evolve a subspecies.