Four Lepidoptera-specific reference strains of Bacillus thuringiensis, belonging to serovars sumiyoshiensis (H3a:3d),fukuokaensis (H3a:3d:3e), darmstadiensis (H10a:10b) andjaponensis (H23), which produce spherical parasporal inclusions, were examined for comparative characterization of δ-endotoxins. SDS-PAGE profiles of the alkali-solubilized parasporal inclusions revealed the presence of single major protein bands of 130 kDa in the four strains. Chymotrypsin and trypsin treatment of the proteins gave profiles different from those of the strains HD-1 (serovar kurstaki, H3a:3b:3c) and T84 A1 (serovar sotto, H4a:4b). Also, minor variations were observed in proteolysis profiles among the four strains. The LC50 values of purified parasporal inclusions for the silkworm (Bombyx mori) larvae were 7·35, 6·45, 3·08 and 2·63µg g-1 diet, respectively, showing that their toxicity levels were 5-15 times lower than that of the strain HD-1 (0·49 µg g-1 diet). Analysis by immunodiffusion and immunoblotting with polyclonal antisera revealed that parasporal inclusion proteins of the four strains are highly related, whereas they shared few or no common antigens with those of the strains HD-1, T84 A1 and Buibui (serovarjaponensis).