Javan gibbons are a monogamous and highly territorial primate species. While inter-group interactions in group living primates have been relatively well documented, they are less studied in gibbons. In this study, we tested hypotheses on wild Javan gibbons (Hylobates moloch) in the Gunung Halimun-Salak National Park, Indonesia, to investigate aspects of inter-group interactions. We focused on the agonistic interactions between the groups to examine four hypotheses, 1) male resource defense, 2) female resource defense, 3) male mate defense and 4) infant defense. Encounters occurred once every two days on average, and agonistic interactions accounted for more than two-third of total encounters. An analysis through Multiple Regressions Analysis revealed that none of the predictor variables (abundance of food, preferred food species or preferred fig, female fertility and presence of dependent offspring) explained the intensity of agonistic inter-group interactions, suggesting that the hypotheses proposed for other primate species are not supported in Javan gibbons, with the parameters used in this study. However, fruit abundance and month were significantly correlated with the intensity of aggressive interaction. Female fertility did not affect inter-group aggression and males directly re-chased males from other groups whenever their partner was chased. On the other hand, affiliative inter-group interaction was hardly found in Javan gibbons, compared to Lar gibbons (H. lar) which engage in social interactions at a relatively high frequency with their neighbors, such as playing, grooming and extra-pair copulation. Playing between juveniles from different groups was observed only once in our study groups. However, playing did not last long as their parents intervened. Based on these results Javan gibbons appear to exhibit different inter-group behaviors, and further study is required.