C-type virus particles and particles, approximately 35 nm in diameter, were present in the region of the basal plate from the placenta of a rhesus monkey and two baboons. Both particles appeared to bud from the plasma membrane of the cytotrophoblast: large, pleomorphic cells with cytoplasmic extensions, indented nuclei, well-developed endoplasmic reticulum, and glycogen deposits. Extracellular particles were enmeshed within a fibrous matrix. Particles were also observed in the junctional zone, but not in the decidua. C-type virus particles from the rhesus monkey and baboons differed in ultrastructure from each other and from C-type mouse leukemia virus particles. The 35-nm-type particle was spherical with a dense central core.