The most conspicuous and species-rich ant-plant mutualism in the Malesian region is found in the important pioneer tree genus Macurungu, yet little is known about the identities or community ecology of the species involved. Our studies have revealed a far more complex system than previously thought. This paper presents the first extensive investigation in the whole distribution area of myrmecophytic Mucarungu. All ant-inhabited species were restricted to the moister parts of SE Asia: Peninsular Malaysia, South and East Thailand, Sumatra and Borneo. We found a rather strict and similar altitudinal zonation of myrmecophytic Macaranga species in all regions. Here we focus on the majority of the 19 Mucaranga species obligatorily associated with ants of the genus Crematogater. We identified a total of 2163 ant queens which belonged to at least eight (morpho)species of the small subgenus Decacraa as well as to one non-Decacrau @robably from Atopogyne). The ant species were not randomly distributed among the Mucuranga species but distinct patterns of associations emerged. Despite common sympatric distribution of Mucurangu species, in most cases a surprisingly high specificity of ant colonization was maintained which was, however, often not species-specific but groups of certain plant species with identical ant partners could be found. These colonization patterns usually but not always mirror existing taxonomic sections within the genus Macaranga. Possible mechanisms of specificity are discussed. The results are compared with other ant-plant mutualisms. 0 1999 The Linnran Socirtv of Ixlndon ADDITIONAL KEY WORDS:-biogeography ~ co-evolution -Decucremuhost specificity~