The social calls of 14 Myotis lucifugus (5 adult males; 5 adult females; 2 subadult males; 2 subadult females) produced under controlled conditions were more variable than echolocation calls made by hunting individuals in the wild. The social calls contained lower frequencies and in some cases were of longer duration than the echolocation calls. Careful examination of sound spectrographs of the calls indicated tremendous variability in details of frequency and time. Although several types of calls can be recognized, a discrete classification is not feasible because of the variability. The possible significance of the variation in the calls of these bats is discussed in the context of recognition of individuals by their vocalizations.