Complete mortality of Simulium damnosum Theobald s.l. larvae was obtained along a 19 km stretch of the Marahoué River including and downstream of the Danangoro rapids complex in the Bandama Basin of Ivory Coast after treatment with 1.5 ppm/10 min of the Sandoz 402-1-WDC formulation of Bacillus thuringiensis Berliner serotype H-14. Partial control was observed for an additional 15 km. The level of control was especially encouraging considering the low concentration (.8%) of active ingredient in the Sandoz formulation. Additionally, the treated population has demonstrated resistance to temephos (Abate), the larvicide currently utilized in the Onchocerciasis Control Programme. Other species of Simulium were somewhat less affected by the treatment; living larvae were found 4 km downstream of the treatment point. Reinvasion of the Danangoro complex, ostensibly by drifting larvae of Simulium spp., was detected the day after treatment. Non-target organisms, including Ephemeroptera and Chironomid midges, were observed before and after treatment and were apparently not affected. In a second test, 0.8 ppm/10 min of the Sandoz formulation was tested, 0.8 pm/10 min of the Sandoz formulation was tested against S. damnosum s.l. and three other Simulium species in the N'Zi River in Ivory coast utilizing a gutter bioassay apparatus. Six hours after treatment 91% of the S. damnosum s.l. larvae had died. The other species responded with 91--100% mortality.