Oyster classification is problematic because of variations in shell morphology, and the use of molecular data may help to confirm species identity and clarify taxonomic confusion. Two ostreid oysters were collected from Peru and identified as Undulostrea megodon (Hanley, 1846) and Talonostrea talonata (Li & Qi, 1994), based on shell morphology. To confirm their taxonomic status, fragments of mitochondrial cytochrome oxidase I and 16S rRNA genes were sequenced. Phylogenetic analyses of both sequences do not support the validity of the two genera that were solely founded on these two species. Oyster U. megodon is clearly a member of Ostrea and T. talonata a member of Crassostrea. Thus, U. megodon should retake its original name of Ostrea megodon (Hanley, 1846), and T. talonata should be renamed as Crassostrea talonata (Li & Qi, 1994). Oyster C. talonata is a described species of China, and its occurrence in western Pacific is unusual and probably because of human facilitated introductions.