The Suminoe oyster Crassostrea ariakensis is being considered for introduction into the middle Atlantic coast region of the United States, where diseases have decimated native stocks of the eastern oyster C. virginica. Triploid C. ariakensis produced at the Virginia Institute of Marine Science (VIMS) and transferred to Bogue Sound, North Carolina, experienced high mortality in the summer of 2003. Histopathological examination of oysters collected 9 d after peak mortality in August revealed the presence of intrahemocytic inclusions, suggesting a Bonamia-like parasite infecting hemocytes in 9.1% (2/22) of the oysters. November sampling of a subsequent October 2003 deployment to Bogue Sound revealed that 47% were infected by Bonamia-like microcells. Diagnosis of a second sample from this group by Bonamia-specific polymerase chain reaction primers revealed 60% prevalence, and subsequent DNA sequence data confirmed that the parasite was a Bonamia. Two samples collected during the peak mortality and archived in North Carolina were received by VIMS in January 2004. These samples revealed 90.9% and 100% prevalence of Bonamia sp., 10 of the 14 infections being heavy in intensity. No infections were detected in C. ariakensis from Pamlico Sound, North Carolina, or from the York River, Virginia, which had originated from the same hatchery cohort, suggesting that the parasite originated in Bogue Sound and was not present in the seed transferred from VIMS. Additionally, no Bonamia sp. infections were detected in C. virginica from Bogue Sound, indicating specific susceptibility of C. ariakensis to the parasite. The natural host for this Bonamia sp. is unknown. How, or if, this parasite will affect the suitability of C. ariakensis for resource restoration or aquaculture in the middle Atlantic region must await studies on the temperature and salinity tolerance, transmission, and distribution of the natural host. Nonetheless, C. ariakensis appears to be highly susceptible to a previously unknown local pathogen.