In a continuation of the World Health Organization's collaborative scheme for testing new insecticides against malaria vectors, field trials were carried out during 1965 in the forest belt of Western Nigeria near Lagos and in the Guinea savanna belt of Northern Nigeria near Kaduna, using bromophos residual spray and cheesecloth impregnated with Bayer 39007; Schering 34615 was applied as a residual spray in Lagos. Although bromophos appeared to be effective for at least five months after spraying in the Lagos area, the desired control was obtained for only one month in the Kaduna area; these differences appear to be due to differences in environmental conditions and to the ecology of Anopheles gambiae. Schering 34615 was also effective for up to five months in Lagos and was definitely superior to bromophos. Cheesecloth impregnated with Bayer 39007 gave good control of mosquitos for two and for three months in Kaduna and Lagos, respectively, but because the walls of the treated huts were only partly covered, some mosquitos avoided contact with the cloth. It is suggested that, because of the reduced toxic hazards, cheesecloth treatments may be feasible with potentially effective insecticides that are too toxic to spray.