Oocysts of Nematopsis mytella, an apicomplexa gregarine parasite of the mussel Mytella falcata (fam. Mytelidae) and of the oyster Crassostrea rizophorae (fam. Ostreidae), were collected from Itamaracá Island and Suape, two distinct zones in the estuarine region of the northeastern coast of Brazil, near the city of Recife. The oocysts were examined by light and transmission electron microscopy. No morphological and ultrastructural differences were observed between oocysts of the two host species. The oocysts were engulfed by the cytoplasm of host phagocytes, with each phagocyte containing 1–19 oocysts (usually 3–5). Each oocyst was located in the center of a spherical to ellipsoidal parasitophorous vacuole (PV) 20–25 μm in diameter. The oocyst (13.2 μm long and 8.4 μm wide) consisted of an external wall 0.6 μm thick surrounded by an internal, uninucleated sporozoite. The apical region of the wall of each oocyst contained a circular micropyle 0.8 μm in diameter covered by an operculum. The oocyst wall was connected to the PV membrane by a complex network of anastomosed microfibrils. The PV matrix contained 1–2 dense bodies and the periphery was occupied by numerous glycogen particles. This is the first record of a Nematopsis sp. from the northeastern coast of Brazil and only the second from Brazil as a country.