Acoustic features of coo calls in Japanese macaques (Macaca fuscata) show large and graded variation. To explore the relevance of acoustic variation in coo calls, I examined whether acoustic features differed by the caller's activity and proximity to group members. The subjects were five adult females from a wild, habituated group of Japanese macaques consisting of 23 individuals. Coo calls from the five females were recorded with their activity and proximity to group members. Acoustic features of 280 calls were measured with a sound spectrograph. Some of the acoustic variables differed by proximity but not by activity. The callers produced coo calls with larger frequency modulation and longer duration when they were far from group members compared to when they were near another member. The results suggest that Japanese macaques produce calls with more detectable and locatable features depending on the proximity to group members. Am. J. Primatol. 69:1412–1424, 2007. © 2007 Wiley-Liss, Inc.