The World Health Organization Onchocerciasis Control Programme in the Volta River Basin of West Africa aims at reducing the transmission of Onchocerca volvulus Leukart, the cause of river blindness, to a level no longer dangerous to health by attacking the larval stage of the vector Simulium damnosum Theo s.1. at its breeding sites in the rivers. The programme covers an area of 700,000 sq. km. embracing 7 countries in which an estimated 1 million people are affected. Up to 14,000 km of rivers are treated with temephos by air using a fleet of 8 helicopters and 2 fixed-wing aircraft. The insecticide is applied at a dosage of 0.05 ppm/10 mins of river discharge. The effect of the applications is monitored on the ground by inspection of the rivers for aquatic stages and by assessing biting rates of black flies by making full day catches at some 270 points each week. After the 1st year of full scale control transmission rates in the center of the area are deemed satisfactory, but on the perimeters dangerous transmission continues. All evidence points to this being caused by flies invading the area from the south and west.