A social parasite uses workers of another social insect species to rear its own progeny. They are often so closely related to their hosts that it has been suggested that they could have evolved sympatrically from them. To address the question of whether social parasites evolved from their hosts we present a partial sequence of the mitochondrial 16S rRNA gene for nine species of Polistes, comprising all known species of social parasites, their hosts and two outgroups. Parsimony and maximum-likelihood analyses of the data support monophyly for these social parasites. The trees supporting monophyly are significantly shorter than the trees supporting sympatric speciation of parasites from their hosts. These data support the hypothesis that speciation occurred allopatrically and independently of the evolution of social parasitism. Where the social parasite parasitizes more than one species, the two species used are most closely related to each other. Although social parasites are monophyletic and did not evolve sympatrically from their hosts, it is clear that relatedness among species is important in the host-parasite relation.