Protomagalhaensia cerastes n. sp. is described from nymphs and adults of the Pallid cockroach, Phoetalia pallida. Gamonts of Protomagalhaensia species are elongate and serpentine in general shape, but associated gamonts of P. cerastes are considerably smaller than those of other species of Protomagalhaensia. Primites and satellites of P. cerastes average total lengths of 323.1 µm and 317.9 µm, respectively; whereas similar stages range from 400.0 µm to 650.0 µm in the other 4 species within the genus. All species of Protomagalhaensia possess dolioform oocysts. Oocysts of Protomagalhaensia granulosae and Protomagalhaensia serpentula also possess apical corner spines or knobs that are absent in the oocysts of Protomagalhaensia wolfi, Protomagalhaensia blaberae, and P. cerastes. The oocysts of P. granulosae possess a lateral depression unique among members of the genus, while P. cerastes and P. wolfi possess distinct polar plates absent in other members of the genus. Oocysts of P. cerastes are notably smaller than those of P. wolfi in both length (7.3 µm vs. 9.2 µm) and width (4.5 µm vs. 5.5 µm).