Man-made treeholes ("tungu") hollowed out from the lower part of the trunk of standing coconut palms are a feature of the Tokelau Islands (tropical Polynesia). Rainwater trapped in them is near neutrality and of low electrical conductivity, and serves as an important permanent source of the Bancroftian filariasis vector, Aedes polynesiensis. From 2-5 June 1978 the population of this mosquito on the islet of Fenuafala, Fakaofo atoll, was exposed to imported Romanomermis culicivorax (eggs in sand cultures). A temporary establishment was confirmed in August 1978 and a monitoring visit in November 1980 revealed that the worm was still causing infections in at least 3 of the tungu initially treated. Subsequently (15 May 1981) these yielded additional parasitized larvae. The establishment had thus persisted for almost 3 years following a single inoculative application. Data presented suggest the future usefulness of R. culicivorax as an element in an integrated mosquito control methodology designed for the Tokelaus.