Two classes of spore mutants have been selected in Bacillus cereus T, those producing lysozyme-sensitive spores, and those producing spores dependent upon lysozyme for germination. One mutant from each class was studied in detail and found to have defective packing of the spore coat layers. The major spore coat poplypeptide appeared to be altered on the basis of gel electrophoretic profiles and/or peptide maps of half-syctine-containing peptides. The spores of the mutants of both classes were sensitive to lysozyme and failed to respond to the germinants L-alanine plus adenosine. The spores were also more sensitive to octanol than the parental strain, but contained the same amount of dipicolinic acid and were equally heat resistant. The reversion frequencies in both cases were consistent with an initial point mutation, suggesting that an alteration in the major coat polypeptide accounted for the phenotypic properties studied.