Badger (Taxidea taxus) activity was recorded in colonies of yellow-bellied marmots (Marmota flaviventris) during behavioral observations and trapping. Badgers were observed seven times in a marmot colony and extensive digging at marmot burrows was recorded five times in 40 y. When four badgers occupied a burrow at the edge of a marmot colony, the behavior of marmots and badgers was observed for 7 d and the marmots for an additional 6 d after the badgers departed. Badgers hunted at a significantly higher rate in the afternoon than in the morning. The adult female usually hunted alone, failed to capture any adult marmots, but did kill young marmots. When badgers were present, marmots had more frequent alarm calls, lower rates of foraging and higher rates of vigilance than when badgers were absent. I estimated that badgers killed 67 of 1423 individual marmots. The risk of being killed by a badger was highest for young, intermediate for yearlings and lowest for adults. Badgers probably have little impact on marmot demography except for localized, short-term increases in mortality, but may significantly affect the fitness of individual marmots.