Endangered black-footed ferrets (Mustela nigripes) are difficult to monitor because of their nocturnal and fossorial habits, but land use and management are influenced by their potential presence. Detector dogs have been suggested as a method for determining ferret presence, although its efficacy has not been thoroughly investigated. We evaluated 2 dogs trained specifically for determining ferret presence in field evaluations conducted in black-tailed prairie dog (Cynomys ludovicianus) colonies at the Conata Basin reintroduction site in South Dakota, USA, during September and October 2003. We tested the dogs on 4 test colonies that had no record of ferret presence and 7 colonies known to have ferrets inhabiting them. One dog was 100% accurate at detecting presence and the other was between 57% and 71% successful at detecting ferrets, with neither dog falsely indicating presence when ferrets were absent. For the 2 dogs, the mean time to detect ferrets on a prairie dog colony was 21 minutes and mean search rate was 26 ha/hour. The mean time to detection on the same sites was 208 minutes for spotlight surveys and mean search rate was 1.6 ha/hour. Although spotlight surveys are necessary for identifying population demographics, well-trained detection dogs show promise for detecting ferret presence in prairie dog colonies.