Electron micrographs of negatively stained regular surface layers (A-layers) of Aeromonas salmonicida showed two square patterns having p4 symmetry. Computer image processing demonstrated that, at a resolution of 2.3 nm, both square arrays were composed of two different morphological units arranged alternatively to give a face-centered lattice in which the four nearest neighbors of each unit were the other type of unit. The lattice constant was slightly but significantly different in the two patterns, and the orientation of one of the two morphological units changed by about 20 degrees between patterns. These patterns were probably not derived from different strains present in the preparation, since both were seen in material that appeared to come from a single layer. This and the difference in lattice constant made it unlikely that they represented different sides of the A-layer. However, it is possible that the two patterns may reflect a structural transformation of the layer. In this respect, it is interesting that the rotation of one morphological subunit changed the size of the gaps between units in the layer. This raised the possibility that the transformation could be related to a change in permeability of the A-layer, possibly analogous to that proposed for gap junctions in eucaryotic cells.