We have used electron microscopy and image processing by computer to study the surface layers from the cell walls of two strains of Clostridia . The glycoprotein subunits in these layers form regular periodic arrays, one with hexagonal symmetry, the other with tetragonal symmetry. Because the lattices tend to be curved or otherwise distorted, a computer procedure for image averaging to reduce noise was developed, which takes account of and corrects the spatial distortions prior to averaging. The average images so obtained show subunits of a size which can be correlated with the known molecular weight of the glycoprotein. The subunits are joined by fine bridges, which provide a good covering of the underlying cell wall.