Taxonomic work has suggested that interspecific hybrids are common in freshwater rotifers and cladocerans. Hybrids within the cladoceran genera Bosmina and Daphnia are apparently common in the large lakes of western Europe (Lieder 1956, 1983a, 1983b; Einsle, 1966, 1978). Similarly, Brooks (1957) noted a number of North American populations of Daphnia which consisted largely or entirely of individuals intermediate in morphology between two recognized species. Additional evidence of hybrid forms has been noted in several rotifer genera (Pejler, 1956). The abundance of hybrid individuals is a striking characteristic of all these examples. Indeed, it has often been noted that presumed parents have been displaced by their "hybrid" offspring (Brooks, 1957). The mechanism by which such displacement occurs is not clear. Moreover, there has been no genetic confirmation that hybridization is actually involved. Daphnia carinata King and Daphnia cephalata King are the two most common Daphnia species in ponds throughout eastern Australia. In some ponds, they coexist with a third, morphologically intermediate, taxon (D. wankeltae Hebert). The three taxa are similar in size, and head shape is the only conspicuous morphological feature distinguishing them (Hebert 1977a). D. cephalata ordinarily has a prominent dorsal crest, which D. carinata lacks, while D. wankeltae has a crest of intermediate size. The present study provides evidence that D. wankeltae has arisen as a result of hybridization between these two species. The genetic data show that introgressive hybridization is not involved. Population surveys indicated that all individuals of D. wankeltae possess a F, hybrid phenotype and suggested that they failed to backcross with their parental forms. However, laboratory studies have shown that the hybrid individuals are not only able to reproduce parthenogenetically, but are also able to produce fertile sexual eggs.