Surmmary ‘Coat fractions’ were prepared from spores of six Bacillus species by mechanical disruption, centrifugation, and digestion with lysozyme. The coats were successively extracted with dilute NaOH, treated sonically (mildly), and digested with the proteolytic enzyme preparation Pronase. Bacillus megaterium QM B1551 was studied in detail. Electron microscopic studies were made at the various stages. Three fractions were obtained and partly characterized chemically. The alkali-soluble fraction consisted mainly of protein(s). The paracrystal fraction solubilized by sonic treatment consisted mainly of protein with a chemical composition and physical characteristics similar to those of keratins. The final resistant residue fraction of the coats after hydrolysis contained amino acids, phosphorus and muramic acid, and may be composed of a phosphomuramyl polymer to which a peptide and/or protein is linked. Electron microscopic observation suggested that the spore coat of B. megaterium QM B1551 consisted of at least three components: a middle, paracrystal fraction is sandwiched between or ‘cemented’ with the alkali-soluble fraction on one side and the resistant residue layer on the other. Large differences were found in the composition of the coats and in the P contents of the resistant residue of the various organisms.