The article is a result of a project titled: The Impact and fate of pesticidal proteins from Bacillus thuringiensis spp. in tropical soils and waters as influenced by interactions with clays - MICROBE - (Project No. INCO no ERBICI8 CT 970135)Laboratory colonies of Anopheles gambiae and Culex quinquefasciatus were reared from parental larvae obtained from the Ifakara Health Research Centre and septic tanks ill Morogoro Municipality respectively. Water ponds (1M3 ) were constructed at selected sites and left open for mosquitoes to breed ill freely. Various concentrations of Bti crystals were-mixed with varying concentrations of clay and tested against third instar larvae of each species at various periods of time post-preparation. A total of 60 larvae (20 x 3) were exposed to each mixture in the laboratory, and each- experiment was replicated four times. Mortality rates were recorded after 241t. exposure. The observations revealed that for every concentration of Bti crystals used, the toxicity on both species of mosquitoes was less persistent in crystal/clay mixtures than in crystals alone. It was also shown that such persistence was much less in mixtures containing large concentrations of clay (5mg/ml) titan in those containing low concentrations (0.05 - 0.5 mg/ml) of clay. It wasfurther observed th at the toxicity against Culex and Anopheles larvae was more persistent in mixtures containing higher concentrations of Bti crystals (0.12 - 0.3 ug/ml) titan ill those containing low concentrations (0.03 - 0.06 ug/ml) of the toxin. Furthermore, it was observed that C. quinquefasciatus larvae succumbed to Bti crystal/clay mixtures for longer periods titan A. gambiae larvae. It was generally concluded that clays have 11 negative impact on the persistence of Bti toxicity against mosquito larvae and th at such impact is more significant in mixtures containing large concentrations clay. It was also concluded that despite the environmental friendliness of Bti, its applicability by communities is limited in view of its short persistence when it is adsorbed to tropical soils which are found in most natural mosquito breeding sites.European Union (EU)