Heat-stable somatic (O) antigens of Bacillus thuringiensis vegetative cells were analyzed by an agglutination test using 23 strains which belong to 17 subspecies. Each O antiserum agglutinated the homologous O antigen at a titer of 512 to 2, 048, and was comletely absorbed by the homologous O antigen. Seventeen B. thuringiensis subspecies were classified into 13 groups by the O agglutination test, i.e., thuringiensis, finitimus, alesti-darmstadiensis, sotto, dendrolimus, canadensis, subtoxicus, enlomocidus, morrisoni-tolworthi, toumanoffii, thompsoni, kurstaki-galleriae-aizawai, and kenyae. Apparent antigenic variation by the O agglutination test was observed in the B. thuringiensis strains of H-serotypes 4a : 4b and 6.