A local isolate of Bacillus thuringiensis (B.t.81) produced crystal protein was identified as a cry I gene of Class I. The synthesis and assembly of crystal complements were investigated at intervals throughout the growth cycle. Incubation temperature had a marked effect on toxin synthesis; production being the highest at 25-degrees-C and the lowest at 42-degrees-C. The mutants of B. t. 81, unable to synthesize crystal protein complements, have also been described.