A phylogenetic analysis of partial sequences of 2 mitochondrial genes, NADH dehydrogenase subunit 4 (ND4) and cytochrome b (cyt b), supports the generic distinction and sister taxon relationship of Platemys and a monophyletic Acanthochelys. Acanthochelys radiolata is the most distinct member of the genus Acanthochelys and is sister to a clade that includes A. spixii, A. pallidipectoris, and A. macrocephala. Parsimony and maximum likelihood analyses suggested A. spixii (A. pallidipectoris, A. macrocephala), but this was not strongly supported in bootstrap analysis parsimony and maximum likelihood analyses and low Bayesian posterior probabilities. Evidence of molecular diversity within morphologically defined species suggests that there may be additional cryptic species in this group.