AbstractInteractions are reported between white‐handed gibbons (Hylobates lar) and pig‐tailed macaques (Macaca nemestrina) in Khao Yai National Park, Thailand, in which gibbons selected ripe fruit from sources before macaques arrived on 4 occasions during June and July of 1989. The macaques foraged near gibbons or from shared fruit sources during 13% of gibbon observation time. Gibbons made their presence known in fruit trees when macaques approached on 3 occasions and macaques did not enter fruit trees occupied by gibbons. An aggressive interaction is reported in which a single white‐handed gibbon vigorously excluded 22–28 macaques from a rare and valuable fruit tree (Sandoricum koetjape). The observations suggest that exploitative and interference competition may exist between these