Bacillus brevisstrains contain a defective prophage. In strain ATCC 10068 this prophage is responsible for the production of particles resembling phage tails and the induction of events leading to cell lysis. There is a slow rate of spontaneous production of particles, which is greatly increased by treating growing cells with mitomycin C, ultraviolet light,N-methyl-N-nitroso-N′-nitroguanidine, and acridine orange or by changing the growth medium. There is no conversion of host DNA to a unique, phage-size DNA entity associated with induction. It was not possible to detect strains that did not produce particles on treatment with mitomycin C. The particles themselves had no recognised biological activity. The molecular weight of the major sheath protein was 49 000 and at least four other protein components were detected in partially purified preparations. DNA and RNA were not associated with the particles but a small amount of carbohydrate was detected.