The effect of penicillin-G on the production of the potential microbial insecticide thuringiensin by Bacillus thuringiensis sp. darmstadiensis was studied. Shake flask and 3-L jar fermenter studies showed that the addition of 360 units/mL penicillin-G at 9 h, when the fermentable sugar in the medium was about to be mostly consumed, improved thuringiensin production by more than 1-fold relative to the control. The dosage of 360 units/mL penicillin-G had only a modest effect on the growth of the microorganism. However, cell growth was inhibited at higher dosages of the antibiotic. Since penicillin-G could interfere with cell wall synthesis, which facilitated the release of thuringiensin, a high thuringiensin productivity of 2600 mg/L was attained in this study, which is about 2-10-fold higher than those values reported in the literature.