Northern grasshopper mice (Onychomys leucogaster) have been implicated as a potential reservoir for plague, which causes local extinction of black-tailed prairie dog (Cynomys ludovicianus) colonies. To identify mechanisms by which grasshopper mice could facilitate plague spread, we measured burrow use, movements, and flea burdens of mice in colonies in northern Colorado. At the microhabitat scale, powder-tracked mice (n = 41) used both prairie dog mounds and burrows extensively, entering an average of 5.3 burrows per 100 m traveled. Burrow use did not differ between active and inactive mounds, or vary with mouse age, sex, or reproductive status. Radiotracking revealed that mice occupied larger ranges (X̄ = 3.84 ha) than reported off colonies, which we estimated would overlap 12–23 prairie dog coterie territories. Mice also harbored high flea burdens (8.1 fleas/mouse), including fleas associated with prairie dogs, which we attributed to their frequent use of burrows. Our results support the contention that, at high population densities observed in colonies, grasshopper mice facilitate plague spread by transporting infected fleas between burrows across prairie dog social boundaries represented by coterie territories.