Fierce boundary fights between Oecophylla smaragdina colonies were previously identified as the major factor limiting ant populations and the efficiency of ants as biological control agents. In order to determine the feasibility and effect ofpreventing boundary fights between colonies, experiments with full-, semi- and no-isolation of existing antcolonies in cashew plantations were done in 1996 and 1997. In a related experiment, ant colonies were transplanted from native vegetation to a cashew orchard. Trees with ant colonies which were fully isolated from other colonies were significantly less damaged by the main insect pests and produced significantly higher yield than those with ant colonies which were partly isolated or were not isolated. That was because fighting events between fully isolated ant colonies were eliminated, and the populations of these colonies were high throughout the cashew flowering and fruiting period. Trees in which O. smaragdina colonies were transplanted suffered little damage by the main insect pests and produced high quality nuts and panicles. However, trees which were protected by pesticides produced lower quality nuts and panicles, because these trees suffered damage by the tea mosquito bug, Helopeltis pernicialis, and the mango tip borer, Penicillaria jocosatrix. It is suggested that O. smaragdina colony isolation, combined with ant transplantation, is an effective means both to achieve high ant populations in cashew plantations and to obtain a high yield.