From synchronized sporulation and spore mutant studies, the order of development-of resistance to biocides during sporulation of Bacillus subtilis strain 168 was toluene, formaldehyde, sodium lauryl sulphate, phenol, phenylmercuric nitrate, m-cresol, chlorocresol, chlorhexidine gluconate, cetylpyridinium chloride, moist heat, sodium dichlorisocyanurate, sodium hypochlorite, lysozyme and glutaraldehyde. These resistances could be assigned to different stages in spore development.