Four strains belonging to Bacillus thuringiensis serovars thompsoni, malaysiensis, canadensis, jegathesan and two auto-agglutinating B.t. strains were identified as being highly toxic to the mosquito larvae of the species Aedes aegypti, Anopheles stephensi, and Culex pipiens. Their larvicidal and hemolytic activities were determined and compared with those of strains known to be highly mosquitocidal and/or cytolytic from serovars of B.t. israelensis, morrisoni, darmstadiensis, medellin, kyushuensis, and fukuokaensis. The electrophoretic protein profiles of purified crystals and immunological relationships with B.t.i. polypeptides were studied. Five out of the six new strains showed the same larvicidal and hemolytic activities and the same crystal proteins and toxin genes as B.t.i. One strain, B.t. jegathesan 367, presented a novel pattern of larvicidal activity and a protein profile different from those of other strains.