Bacillus thuringiensis var. kyushuensis synthesizes an irregularly shaped parasporal inclusion during sporulation. Electron microscopy revealed that the inclusions are composed of a relatively homogeneous appearing center surrounded by a thick, electron dense coating. Purified inclusions were found to have an LC50 of 12 ug/ml. against Aedes aegypti larva which is nearly 400 times less toxic than similar preparations of B. thuringiensis var. israelensis inclusions. The inclusions are composed of four major peptides with molecular weights of about 140 kDa, 70 kDa, 26 kDa, and 14 kDa. Blots of the inclusion peptides were prepared and probed with monoclonal antibodies specific for the three major peptides present in B. thuringiensis var. israelensis inclusions. Antibody directed against the 135 kDa B. thuringiensis var. israelensis peptide was found to cross react strongly with the 70 kDa peptide of B. thuringiensis var. kyushuensis. The antibody directed against the 65 kDa peptide of Bacillus thuringiensis var/ israelensis was found to strongly react with the 26 kDa peptide of B. thuringiensis var. kyushuensis. However, the antibody directed against the 28 kDa peptide of B. thuringiensis var. israelensis failed to react with any of the B. thuringiensis var. kyushuensis peptides.