Formulations of Bacillus thuringiensis serotype H-14 were evaluated in the laboratory for larvicidal potency against the Afro-tropical malaria vector Anopheles arabiensis. The 3 formulations tested were: standard IPS-78 water-dispersible powder; an experimental water-dispersible powder (ABG-6108); a water-dispersible concentrate (San 402-1). Mortality was recorded for batches of 25 second instars exposed for 24 hr, giving LC50 values of 0.159, 0.211 and 0.163 ppm, and for 48 hr giving LC50 values of 0.113, 0.119 and 0.099 ppm, respectively, for the 3 formulations. When compared with the IPS-78 standard wdp the relative activity at LC50 level was 0.75 for 24 hr and 0.94 for 48 hr exposure to ABG-6108 wdp; 0.97 for 24 hr and 1.14 for 48 hr exposure to San 402-1 wdc. It is concluded that wdp formulations are less efficacious than emulsion which has prolonged availability for ingestion by surface-feeding Anopheline larvae.