Fleas were collected from white-tailed prairie dogs (Cynomys leucurus) and other small mammals trapped on six grids during a field study near Meeteetse (Wyoming, USA) in 1989 and 1990 to investigate the dynamics of plague in this rodent population. Fleas were identified and tested for Yersinia pestis by mouse inoculation. Yersinia pestis-positive fleas were found on prairie dogs and in their burrows. Flea species on prairie dogs changed from spring to late summer. White-tailed prairie dog numbers were significantly lower in the presence of Y. pestis-positive fleas; however, affected populations generally recovered 1 to 2 yr following absence of detectable plague. Grids where recovery occurred had a high proportion of juvenile male prairie dogs. Eighteen flea species were identified on small mammals, six of which were infected with Y. pestis. Some flea species were associated with a particular small mammal species, while others were found on a broad range of host species. Flea species most important in the potential interchange of Y. pestis between associated small mammals and white-tailed prairie dogs were Oropsylla tuberculata cynomuris, Oropsylla idahoensis, and Oropsylla labis. Plague cycled through the white-tailed prairie dog complex in an unpredictable manner. Each summer the complex was a mixture of colonies variously impacted by plague: some were declining, some were unaffected by plague, and others were recovering from plague population declines. These data provide insight into the dynamics of plague in white-tailed prairie dog complexes, but predicting movement of plague is not yet possible and the role of associated mammals in maintenance of plague is not understood.