Parapharyngodon grismeri n. sp. from the intestines of the central Baja California banded rock lizard, Petrosaurus repens, and the San Lucan banded rock lizard, Petrosaurus thalassinus, is described and illustrated. Parapharyngodon grismeri n. sp. is the forty-second species assigned to the genus and differs from other species in the genus in that males possess posteriorly situated lateral alae, have cloacal lip adornment, and have spicules 116–128 µm long; females possess alate eggs and a stout terminal spike. In addition to the new nematode species, gravid individuals of Oochoristica sp. (Cestoda: Linstowiidae), Spauligodon giganticus (Nematoda: Pharyngodonidae), and Strongyluris similis (Nematoda: Heterakidae) and larvae of Ascarops sp. (Nematoda: Spiruridae) and Physaloptera sp. (Nematoda: Physalopteridae) were found in P. repens, and S. giganticus, S. similis, and larvae of Physaloptera sp. were found in P. thalassinus.