The harvester ants Pogonomyrmex barbatus and P. rugosus serve as hosts for the parasitic ants P. anergismus and P. colei. Recent molecular studies demonstrated that morphology is unreliable for identification of hosts because the hosts contain cryptic species. Using mitochondrial-DNA sequences, we determined identity of 15 of 16 known populations of hosts, and in 9 instances, the identity of the particular host colony. Hosts are composed of 6 reproductively isolated lineages. While P. colei is known only from a single host, P. anergismus occurs in five distinct hosts and appears to have a much broader geographic range. We report seven new locality records for P. anergismus. Range extensions for P. anergismus in the United States consist of southwestern Texas including Big Bend National Park, northwestern Texas, central New Mexico, and south-central Arizona. We discuss factors that may influence dispersal of parasitic ants and host-parasite co-evolution.