Two species of gastric-brooding frogs, Rheobatrachus silus and R. vitellinus, currently constitute the subfamily Rheobatrachinae of the Australian leptodactyloid family Myobatrachidae. The relationships of Rheobatrachus to other myobatrachids have remained obscure due to the specializations for aquatic life and unique gastric-brooding behavior of Rheobatrachus and to the rare and endangered status of R. silus, until recently the only known species. An antiserum to the serum albumin of R. vitellinus was used in micro-complement fixation analyses comparing R. vitellinus to R. silus, and to representatives of most of the myobatrachid genera as well as to select representatives of the South American and South African leptodactyloid fauna. The two species of Rheobatrachus are each others closest relative and no other lineage is distinctly associated with these two species. Albumin comparisons involving other leptodactyloids show that Rheobatrachus is part of the Australian fauna, but as all of the major lineages appear to have arisen in the late Cretaceous, no single sister lineage to Rheobatrachus can be unambiguously identified.