In this report, we describe the ultrastructural morphology of rickettsia–like organisms (RLOs) present in gill epithelial tissue of the oyster, Crassostrea rizophorae, from the estuarine region of the Parnaíba river, on the northeastern Atlantic coast of Brazil. Numerous rod-shaped RLOs formed microcolonies that were located in intracytoplasmic vacuoles up to 85 μm in diameter. These RLOs, which measured about 2 μm x 0.6 μm, had ultrastructural characteristics of prokaryotes that included a plasma membrane and a thin, Gram-negative type cell wall. Some nondividing RLOs had a transversal constriction indicative of binary fission. The disappearance of the apical cilia with concomitant lysis of the gill epithelial cells close to the parasitized cells suggested that the RLOs may be associated with mortality in these oysters. Numerous free RLOs were seen following disruption of the vacuoles during host cell necrosis and degeneration. This is the first description of a Rickettsiales-like organism in the Atlantic fauna of Brazil.