A captive-born juvenile female rhesus macaque (Macaca mulatta) was acquired from a commercial breeder and placed in quarantine. Within 8 days of arrival, the animal became anorexic, inactive, and dehydrated. Subsequently, generalized edema and facial ecchymoses developed, and despite supportive therapy, the animal became moribund and was euthanatized. Macroscopic examination showed diffuse stippling and streaking of the myocardium. Histopathologic examination revealed multifocal to coalescing myocardial edema, necrosis, lymphohistiocytic inflammation, and generalized endothelial infection with Sarcocystis sp. Immature and mature schizonts within endothelial cells were most prevalent in the heart. Fewer schizonts were present in the vasculature of other tissues, including skeletal muscle, smooth muscle, adipose tissue, brain, and retina. Mature tissue cysts within muscle fibers were not found in the myocardium but were occasionally seen in skeletal muscle. Analysis of polymerase-chain-reaction-amplified 18s ribosomal RNA gene sequences revealed 96% identity to published sequences of S. hirsuta, S. hominis, and S. fusiformis and 95% identity to S. cruzi and S. tenella. However, sequences did not show complete identity with any organism in the GenBank database. Sequence homology suggests that this is a newly described Sarcocystis sp. Results of antibody tests for simian retrovirus, simian T-lymphotropic virus 1, and simian immunodeficiency virus were negative, suggesting that viral immunosuppression was unlikely to have augmented the pathogenicity of sarcosporidial infection. Clinical and histopathologic findings in this case of fulminant sarcosporidiosis are similar to those described in Dalmeny disease in cattle, which is associated with ingestion of massive numbers of infective Sarcocystis oocysts.