Bacillus brevis 47, a protein-secreting bacterium, contained two major proteins with approximate molecular weights of 150 000 and 130 000 in the cell wall. The cell surface was covered with a hexagonally arranged array of six structural units about 4 nm in diameter with a lattice constant of 14.5 nm. The regular array structure as well as the chemical composition of cell envelopes remained the same regardless of the growth conditions. A mutant, strain 47-57, which was isolated as a phage resistant colony, contained only the 150 000 protein as a major cell wall protein. Although the mutant had hexagonally arranged arrays with the same lattice constant as that of wild-type cells, the distribution of mass in the unit cell differed considerably from that of the wild-type cells. The number of structural units in the unit cell of the mutant was reduced from six to three. Taking these results together with filtered images of the wild-type and mutant envelopes, two possible models for the surface array of B. brevis 47 are discussed.