Parasites were found in red blood cells of two long-tailed macaques (Macaca fascicularis) imported from Indonesia and housed in the Washington Regional Primate Research Center breeding colony for 7 years or longer. Both macaques developed parasitemias secondary to stress (type D retrovirus in one case and severe trauma in the other). Entopolypoides macaci (Babesiidae) was diagnosed on the basis of morphology from peripheral blood smears stained with Wright's stain. Antibodies against Babesia sp. were detected by immunofluorescence assay (IFA) from one infected macaque, which showed antibody cross-reactions (high titer) to B. bigemina, B. bovis, B. canis, and (low titers) to Plasmodium falciparum. Five feral long-tailed macaques that had been imported recently from the same country had no detectable antibodies. This is the first report of IFA as an aid to diagnose E. macaci in nonhuman primates.