Abstract
Cysts of Sarcocystis were observed in 24% (12/51) of laboratory Rhesus monkeys, a relatively higher prevalence than generally reported. Infestation was slight, but was similar in all muscles studied. In view of this, sampling of large blocks of muscle seemed necessary to demonstrate the high prevalence. The fine structure of the cyst wall, an important criterion in species definition, was described. Details of the cyst wall might be misinterpreted at the light microscopic level because of the effects of muscle stretch and angle of sectioning.
