Electron microscopic examinations ofBacillus subtiliscells revealed that relatively high concentrations of carbon sources blocked sporulation at stage 0 in most cells. Both nalidixic acid and novobiocin blocked sporulation at stage 0. The cells treated with acridine orange showed the morphology of stage IV 5 h after the end of exponential growth, but no further progression was observed. Mutants that are able to sporulate in the presence of these agents had the characteristic morphological changes observed in uninhibited cultures.