Infection with Sarcocystis is common in many species of wild cervids but none is reported from the black-tailed
deer (Odocoileus hemionus columbianus). Here, we report Sarcocystis infection in two black-tailed deer from
northwest USA for the first time. Sarcocysts were microscopic, up to 556 μm long and mature. The sarcocyst wall
was up to 1.39 μm thick, and had rectangular 1.17 μm long villar protrusions, type 17, with thin (230 nm) electron
dense ground substance layer. Molecular characterization and phylogenetic analysis indicated that Sarcocystis in the
black-tailed deer is related to structurally distinct Sarcocystis species in cervids. A new name, Sarcocystis
mehlhorni, is proposed for the Sarcocystis species in black-tailed deer.