Conflicting phylogenies have been proposed for the Chelidae (Testudines: Pleurodira), a family of side-necked turtles found only in Australasia and South America. Sequence data from the mitochondrial 12S rRNA gene were used to test these phylogenies. In total, 411 nucleotides were sequenced for each of 16 chelid species, including all 11 recognized chelid genera and, as outgroups, 5 genera of Pelomedusidae (Testudines: Pleurodira). Analyses using parsimony and neighbor joining algorithms strongly support the division of Australian Chelidae into the three monophyletic groups initially suggested by Burbidge et al. (1974; Copeia 2: 392-409): Chelodina (bootstrap value 99%), the Emydura group (87%), and Pseudemydura. The analyses suggest that the Australian chelids are a monophyletic lineage (64%), with the Australian long-necked turtles, Chelodina, more closely related to the Australian short-necked chelids than to the long-necked South American species. These relationships are in contrast to those of Gaffney (1977; Am. Mus. Novitates 2620: 1-28). The species of Australian long-necked chelids consistently form a monophyletic clade, with Chelodina longicollis and Chelodina oblonga as sister taxa. The data failed to resolve relationships among the Australian short-necked taxa: Emydura, the Elseya latisternum group, the Elseya dentata group, Rheodytes, and Elusor. Unlike Gaffney (1977), we find some weak support (58%) for Pseudemydura as the closest relative of the other Australian short-necked taxa. With the exception of Hydromedusa, the South American taxa are monophyletic and the subgenera of Phrynops are paraphyletic.