The LC50 value of alkali-solubilized parasporal inclusion proteins of a Diptera-specific strain, belonging to Bacillus thuringiensis serovar canadensis, was 2.4 mu g/ml for larvae of the mosquito, Aedes aegypti. A significant loss in larvicidal activity occurred when solubilized inclusion proteins were treated with A. aegypti larval gut extract, silkworm (Bombyx mori) larval gut juice, and the proteinase It. Approximately 90% of the larvicidal activity tvas destroyed upon treatment with proteases in 30 min. The parasporal inclusion was composed of major proteins of 65, 53, and 28 kDa and some other minor proteins, Proteolysis profiles showed that the 65-kDa major protein is highly sensitive to proteases. Purification experiments with DEAE-Toyopearl column chromatography revealed that the 65-kDa protein is responsible for the mosquitocidal activity of this strain. The LC50 value of the purified protein was 5.4 mu g/ml.