Black-footed ferrets (Mustela nigripes) excavate soil from prairie dog (Cynomys spp.) burrows, thereby creating characteristic soil deposits at burrow openings. These soil deposits have been observed only rarely in summer. We monitored adult ferrets during JuneOctober of the years 2007 and 2008 on a 452-ha colony of black-tailed prairie dogs (Cynomys ludovicianus) in the Conata Basin, South Dakota. We located and identified ferret excavations during nighttime spotlight surveys for ferrets and daytime sampling of prairie dog burrow openings around locations where ferrets were located via spotlight. We accumulated 48 observations of in-process or recently completed ferret excavations during spotlight surveys (21 in 2007, 27 in 2008) and located 51 diggings during daytime burrow sampling (25 in 2007, 26 in 2008). We located diggings during 5.5% of spotlight observations, most frequently in JulyAugust. These results collectively suggest ferrets may frequently excavate soil in summer, because prairie dogs frequently use soil to plug burrow openings and tunnels in defense against ferrets. Prairie dogs might frequently destroy soil deposits left by ferrets during summer, thereby reducing detection of diggings by biologists.