A microorganism of uncertain taxonomy was discovered in 1963 by J. G. Mackin in association with lesions and mortalities of Japanese oysters, Crassostrea gigas, from Denman Island, British Columbia, Canada. Mackin coined the term "microcell" for this organism, the parasite Mikrocytos mackini sp. n. Similar appearing organisms were seen by the senior author in flat oysters, Ostrea edulis, from Milford, Connecticut, on three different occasions. The causative organism in these three episodes has been shown by electron microscopy to be Bonamia ostreae, the parasite that was implicated in recent mortalities in flat oysters in Europe. Similar organisms have also been seen in Olympia oysters, Ostrea lurida, from Oregon and in the Sydney rock oyster, Saccostrea commercialis, from Australia. Presence of the organism in the latter species is associated with winter mortalities and is here described as Mikrocytos roughleyi (sp. n.).