Bonamia ostreae is a parasitic protist in the phylum Haplosporidia that can cause lethal infections in shellfish, particularly the European flat oyster, Ostrea edulis. Infection in oysters rarely results in clinical signs of disease and often the only indication of the infection is increased mortality. The Australian flat oyster, Ostrea angasi, has been infected with two similar Bonamia parasites, Bonamia exitiosa and B. roughleyi.