To identify the actual mosquito larvicidal toxin(s) in Bacillus thuringiensis serovar kyushmensis, we conducted a comparative toxicity study by a combination of ion-exchange chromatographies and SDS-PAGE. Among the proteins contained in solubilized and protease-treated inclusions, only the 23-kDa protein, a cytolysin referable to the CytB protein, exhibited mosquitocidal activity. The LC(50) value of this protein was 51.4 mu g/ml for the third-instar larvae of the mosquito, Aedes aegypti. The fractions of other inclusion proteins showed no mosquito larvicidal activity at protein concentrations up to 1000 mu g/ml.