Cladistic analysis of 25 species of Dolichovespula and Vespula (yellowjackets) is used to investigate the evolution of social parasitism in these genera. Three species of yellowjackets are social parasites, or inquilines; that is, lacking a worker caste, and dependent on usurping the colony of a host species to obtain a worker force. Emery's Rule states that social parasites are more closely related to their hosts than to any other species. By investigating the phylogenetic relationships among the parasites and their hosts using cladistic analysis, we attempted to determine if Emery's Rule applies to yellowjackets, as is thought to be the case for the ants on which the theory was based. Sixty-eight morphological and behavioral characters are presented to resolve the phylogenetic relationships among these species. Cladistic analysis does not support Emery's Rule, because social parasites are not more closely related to their hosts than to any other species.