Penicillin at concentrations non-inhibitory to the vegetative growth was found to inhibit sporulation in Bacillus polymyxa 2459. The effect of penicillin was shown to be at the level of spore-specific mucopeptide synthesis. Penicillin had no effect on the early events such as DNA and protein synthesis in sporogenesis The sensitive period of inhibition was between T0 to T2 hours of sporulation.