Bacillus cereus French & French increased the mortality of 3rd-instar gypsy moths, Lymantria dispar (L.), caused by Bacillus thuringiensis subsp. kurstaki Berliner. B. cereus did not cause mortality of L. dispar when applied alone. The activity of various B. cereus strains was correlated positively with their accumulation of zwittermicin A, an aminopolyol antibiotic, in culture. When a constant dose of B. thuringiensis subsp. kurstaki and increasing concentrations of purified zwittermicin A were applied to artificial diet, mortality of larvae was directly proportional to the dose of zwittermicin A. Zwittermicin A by itself caused no mortality at the concentrations tested. Addition of zwittermicin A to a culture of a mutant strain of B. cereus that does not accumulate zwittermicin A restored synergistic activity. These results indicate synergy between B. thuringiensis subsp. kurstaki and zwittermicin A. The potential for enhancing efficacy of B. thuringiensis and delaying development of insect resistance are discussed.