Cattle (Bos taurus) are intermediate hosts for three named species of Sarcocystis, S. cruzi, S.
hirsuta, and S. hominis. Recently, a fourth species was identified and named S. sinensis.
However, S. sinensis was originally named for a species of Sarcocystis in water buffalo (Bubalus bubalis) in China. Based on unverifiable evidence, it was suggested that the same parasite infects
cattle. Additionally, S. sinensis was recently declared as nomen nudum because its naming
violated the rules of ICZN. Thus, the fourth species using cattle as an intermediate host does not
have a valid name. Here, we propose a new name, Sarcocystis rommeli for the S. sinensis–like
parasite from cattle in Argentina, and differentiate it ultrastructurally from S. hominis sarcocysts
from experimentally infected cattle. Sarcocystis rommeli sarcocysts were microscopic with a 5
μm thick wall with slender villar protrusions (Vp); the Vp were up to 5 μm long, up to 0.5 μm
wide, and of uneven thickness, often bent at an angle. The ground substance layer (Gs) was up to
0.8 μm thick and smooth. Vesicular structures were seen at the base of the Vp. The bradyzoites
were 10-12 μm long. Sarcocystis hominis sarcocysts had Vp that were often upright, up to 7.5
μm long, and up to 1.8 μm wide; the Gs was up to 2 μm thick and without vesicles. Its sarcocyst
wall was up to 5.6 μm thick, the vp were bent at an angle, up to 5.8 μm long, the Gs was up to 2
μm thick, but without vesicles seen in S. rommeli. The Sarcocystis described here is molecularly
different than S. cruzi, S. hirsuta, and S. hominis based on 18S rRNA and cox1 gene sequences.