This study had three purposes. The first was to examine the relationship between fruit abundance and party size of bonobos. This is the first study at Wamba that provides quantitative data on fruit production and party size for a period of more than a year. As mentioned below, bonobos of our study group had not been artificially provisioned for 7 years when we started observation for this study. Therefore, our study provides valuable information on the ecology of bonobos under natural conditions. The second purpose was to provide data for comparative studies between chimpanzees and bonobos by using the same definition of party size and fruit production. We employed methodologies that had been developed for studies on chimpanzees in the Kalinzu Forest, Uganda (Furuichi et al. 2001, Hashimoto et al. 2001), which would allow for accurate interspecific comparison of the relationship between party size and fruit production. The third purpose was to reevaluate the results of studies on bonobos at Wamba Since 1976 when all members of group E, which split into the E1 and E2 groups before 1983, were identified, they were provisioned with artificial food until 1996 when the study was interrupted by civil wars (Furuichi 1989, Kano 1992). Because the bonobos had been given only a small amount of artificial food for a limited time, researchers working at Wamba assumed that the tendencies observed in E1, such as large and stable party size and gregariousness of females, reflected the nature of wild bonobos in an unbiased way. However, there is reason to challenge that these tendencies may have appeared due to the influence of the artificial provisioning. By comparing current and past tendencies of grouping patterns, we may be able to evaluate the extent to which artificial provisioning influenced past studies at Wamba.