Electrophoretic analyses of six red cell proteins and three serum proteins of Cercopithecus (= Erythrocebus) patas provided evidence for polymorphic or variant forms of the proteins at four genetic loci: carbonic anhydrase I, carbonic anhydrase II, lactate dehydrogenase, and albumin. There was no evidence for the presence of variants or polymorphisms at five loci: hemoglobin α chain, hemoglobin β chain, malate dehydrogenase, haptoglobin, and transferrin. About 30% of the loci tested in C. patas are polymorphic, and this value is in accord with results obtained in other animal species.