Data on the prevalence and distribution of species of Anopheles in Ariquemes (State of Rondonia) show, that the diversity is greater at the periphery of the town and that Anopheles darlingi is recorded at nearly all of the sampling points. The entomological survey revealed different levels of penetration by A. darlingi in to the urban area: Sectors 1 and 3 are free of malaria transmission, Sectors 2 and 4 show risks at the periphery, and the Industrial Sector, Sector of Special Areas, BNH housing estate, Sector 5 and the Old Town are areas of high risk for malaria. These areas had the highest indices of mosquitoes per man-hour, with variations of this index during the sampling period and depending on locality. Measures of population density showed seasonal variation, with the lowest indices during the rainy season. Malaria transmission is discussed in relation to 1) the physical structure of the town at the time it was jounded ; 2) the streams that border the urban area and their relation to the life cycle of the anophelines; 3) biting activity of the species colected in natural environments and areas altered by man; and 4) the importance of environmental management for malaria control by reducing the vector population density. To reduce the penetration of urban areas by mosquitoes from the surrounding countryside, a protective belt around the town is proposed. This belt should be of sparse forest and include alternative hosts to encourage the zoophilic tendencies of the anophelines. Infection rates among anophelines in areas authocthonous for malaria incriminate A. darlingi as a vector. The possibility of transmission by other species is discussed.