Acoustic variability in the species-typical long-distance calls of male chimpanzees was investigated. Analysis of 13 acoustic features revealed that calls varied more within than between individuals. Although only a limited amount of acoustic variability was attributable to different populations, discriminant-function analysis confirmed that males from two populations produce acoustically distinguishable calls. Additional investigation suggested that social factors may contribute to the patterns of acoustic variation both between and within individuals. Matrix-permutation tests revealed a positive association between the amount of time males spent together and a measure of call similarity. Vocal similarities between individuals appeared to result from a dynamic process involving chorusing behaviour: males matched the acoustic characteristics of each other's vocalizations when calling together. Chorusing may also affect the degree of within-individual acoustic variation in calls. Males who chorused often with others produced more variable calls than individuals who chorused less often or called alone.