Field surveys were conducted to: (a) examine the traits of Tetraponera tessmanni and/or its host that enables this ant to monopolize mature lianas; (b) identify insects which occupy Vitex thyrsiflora at different development stages and how they interact with T. tessmanni; and (c) determine if the ant protects its host. The study was conducted in a forest on the slopes of Mont Kala in southern Cameroon. In young individuals of the obligate myrmecophytic liana V. thyrsiflora several species of ants and other arthropods compete for resources offered by the plant. In mature individuals the only inhabitant is the ant species T. tessmanni which is completely restricted to Vitex lianas as its sole host. Established colonies of this ant provide effective defence against herbivores. The association between V. thyrsiflora and T. tessmanni is unusual in 2 respects. First the climbing life form is rare among myrmecophytes. Secondly it is surprising that a pseudomyrmecine should be the obligate associate of a liana. Pseudomyrmecine plant-ants often prune vegetation contacting their host plant. This behaviour functions in part to protect against invasion of the host by ecologically dominant ants. In contrast T. tessmanni does not prune and is associated with a plant whose success and thus that of its resident ant colony depends on contacts with many other plants. Several traits of V. thyrsiflora and T. tessmanni combine to make the colonization of host plants by potential competitors very difficult. These include behavioural and morphological filters restricting entrance into the plant and exploitation of the resources it can supply; plant anatomical organization that enables T. tessmanni workers to carry out all activities except leaf patrolling within a single branched private nesting space within which all food resources offered by the plant are produced; and polygyny permitting the colony to monopolize a large rapidly growing and long-lived territory.