Three strains ofCardiobacterium hominis(31) were studied with the electron microscope. Thin sections showed slender rods with a cell wall of the coherent Gram‐negative type and with a cytoplasm containing numerous intrusive membranes disposed around the periphery of the cells, especially at the poles. The cell wall consisted of a unit membrane sandwiched between dense outer and inner layers, the outer dense layer showing a 4–5 nm thick regular structure. Polar caps of a dense tufty material were noted on the ends of cells. The substructure of the surface layer was studied in negatively stained cell wall preparations obtained after ultrasonic treatment. It consisted of a close packing of nearly spherical units (averaging 3.5 nm in diameter and with a repeat frequency of 5.5 nm) in a rectangular array; it was also demonstrated after freeze‐etching. This layer represents an unusual type of surface structure on a Gram‐negative ba