Sporolithon (Sporolithaceae, Corallinales) is represented in southern Australia by S. durum (Foslie) Townsend et Woelkerling, comb. nov. The species and the development of male and female-carposporangial conceptacles and tetrasporangial compartments and sori are described in detail for the first time. Information on etymology, collections examined, distribution, seasonality and habit and comparisons with other species is also included. Published records of S. erythraeum from southern Australia have not been substantiated. Two further species have been reported under the generic name Archaeolithothamnion. Archaeolithothamnion australasicum is a species of uncertain status, while A. mirabile is conspecific with Lithothamnion muelleri Lenorrnand ex Rosanoff. Differences between the Sporolithaceae and the Corallinaceae, and the delimitation of genera within the Sporolithaceae, are reconsidered in relation to new data on S. durum. The Sporolithaceae is characterised by tetrasporangia that produce cruciately arranged spores and develop within calcified sporangial compartments, while the Corallinaceae is characterised by tetrasporangia that produce zonately arranged spores that do not develop in calcified sporangial compartments. Two genera of Sporolithaceae are recognised: Heydrichia, in which tetrasporangial compartments are enclosed by modified filaments to form sporangial complexes; and Sporolithon, in which tetrasporangial compartments are not enclosed by modified filaments and sporangial complexes are absent. The concepts of conceptacle and sorus also are reconsidered.