The Onchocerciasis Control Program of the World Health Organization is carrying out an extensive screening program in a search for new larvicides to be used for control of Simulium damnosum s.l. Emphasis has been given to finding a pyrethroid and a carbamate to supplement the organophosphates currently in use. These chemicals with differing modes of action, together with Bacillus thuringiensis H-14, are being used in an attempt to cope with the development and spread of resistance to the organophosphates temephos and chlorphoxim.