Habitat loss, introduced disease, and government-sponsored eradication programs have caused population declines in all 5 species of prairie dogs. Black-tailed prairie dogs (Cynomys ludovicianus) currently occupy only about 2% of an extensive geographic range (160 million hectares) and were recently considered for listing under the United States Endangered Species Act. Accurate estimates of density for populations of prairie dogs would be valuable for estimating range-wide abundance and for determining threats to species persistence, yet estimates of density using robust approaches (e.g., complete enumeration or mark-recapture) are rarely undertaken. We introduce a novel approach to estimating density of prairie dogs using mark-resight methods. Using mark- resight, mark-recapture, and 3 other indices, we estimated the abundance of prairie dogs on 3 reintroduced colonies over a 3-year period (2003-2005). We show that mark-resight is a superior approach to estimating abundance of prairie dogs, that average density estimates from the southern extremity of the species' range are considerably lower (11.3 prairie dogs/ha) than estimates from more northerly climes ("x bar" = 1/4 18.3-90.3 prairie dogs/ ha), and that population densities can fluctuate widely in accordance with local environmental conditions. We propose that resource agencies use mark-resight methods to obtain density estimates of prairie dog populations within diverse ecoregions, and couple these estimates with an assessment of the area occupied by prairie dog colonies to determine range-wide abundance.