Comparison of the dental morphology of the Middle Paleocene paromomyid primate Plesiolestes problematicus with that of the Early Eocene microsyopid Cynodontomystatideas indicates that Plesiolestes and Cynodontomys are closely related and that the Microsyopidae are derived from paromomyid primates. The Microsyopidae are, therefore, considered to be primates also. The diagnostic primate molar morphology apparently evolved before the petrosal bulla characteristic of most primates was acquired. Evidence presented here supports derivation of Primates a from leptictid insectivores.