Separate black-tailed prairie dog, Cynomys ludovicianus (Ord), towns on the Rocky Mountain Arsenal National Wildlife Refuge, Colorado, were treated with technical pyriproxyfen (Nylar) using spray, powder, and oral bait carriers. Direct combing methods (1997 and 1998) and burrow flagging (1998) were used to estimate relative abundance of the plague vector Oropsylla hirsuta (Baker). Pyriproxyfen spray (0.05%) and powder (0.05%) did not significantly reduce (P > 0.05) O. hirsuta abundance. Pyriproxyfen bait, when applied every 4 wk at a concentration of 286 mg/50 g bait, significantly reduced (P ≤ 0.05) O. hirsuta infesting prairie dogs, 4 mo after initial treatment. However, flea populations had recovered to pretreatment levels by the following summer (July 1999).