Many species of Colocasiomyia (Diptera, Drosophilidae) depend exclusively on flowers of Araceae species for mating, oviposition and larval development. These flies play an indispensable role in pollination of their host plants. This resembles fig and fig wasp system in having features of both plant-herbivore and plant-pollinator relationships. Characteristic of this pollination mutualism is that fly larvae do not eat the seeds. Furthermore, plant-insect relationships range widely, from obligate mutualism to broad generalism. However, there is some correspondence in host selection between Araceae tribes and Colocasiomyia species-group. Another feature of this pollination mutualism is ‘synhospitalism’, in which two Colocasiomyia species coexist in a single host inflorescence. Usually, one fly species in a synhospitalic pairs uses mainly the upper (male) part of the inflorescence and the other species the lower (female) part. A phylogenetic tree of the flies suggests two possible pathways for the coevolution of this synhospitalism. We review this pollination mutualism and present new findings from Borneo.