A new species of Nematopsis (Apicomplexa, Porosporidae) is described from the mantle tissues of the seawater gastropod, Nerita ascencionis (Neritidae), collected in the Atlantic North off the coast of "Fernando de Noronha" Island (3 degrees 47' 57'' S, 32 degrees 25' 12'' W) situated about 350 km from the northeast coast of Brazil. Numerous oocysts, each contained in a parasitophorous vacuole, were found in the cytoplasm of phagocytes in the mantle tissue of the host. The phagocytes were surrounded by a thin wall composed of lucent material. The phagocyte cytoplasm contained a nucleus surrounded by numerous vesicles and some dense masses. The oocysts were 21.9 +/- 0.5 microm long, and 11.5 +/- 0.6 microm wide. The oocyst wall was 0.18-0.25 microm thick, and the apical zone contained a micropyle, 1.0-1.2 microm in diameter, covered by a canopy-like operculum about 0.25 microm thick. Externally, the oocyst wall was surrounded by numerous anastomosing microfibrils attached to the wall and extending towards the periphery of the parasitophorous vacuole. Some microfibrils formed a dense complex network that surrounded the oocyst in the middle of the parasitophorous vacuole, which opened only at the apical zone near the external region of the opercular system. On the basis of the data obtained by light and transmission electron microscopy and host specificity, the gregarine Nematopsis gigas is distinguished from the nearest species as a new species. The taxonomic affinities and morphological comparisons with other similar species of the same genus are discussed.