The fine structure of the superficial layers of the tablet-forming micro-organism, Lampropedia hyalina, has been studied by several electron microscopical techniques. The outer envelope is distinct from the cell wall and surrounds not individual cells but groups of cells (tablets). This envelope appears to embody two complex layers, both possessing a regular-patterned structure. One layer (the perforate layer) is a honeycomb network of hexagonally distributed holes, each about 75 angstrom in diameter, and with a repeat spacing, from centre to centre, of 145 angstrom. The other, outermost (punctate) layer, which seems to be fragile and easily lost, is composed of outward-projecting spines, also in hexagonal array but with a repeat spacing of 260 angstrom. The probable structural relationship between the perforate and punctate layers is discussed. The structured envelope is separated from the cells by an amorphous zone (the intercalated zone) about 600 angstrom thick; this penetrates between the cells of a tablet and follows immediately behind the ingrowing septum in cell division. The intercalated zone is probably the important factor in cell cohesion; the significance of the structured envelope is uncertain but its function may be to separate the sheet into tablets by preventing intercell cohesion at certain places in what would otherwise be a continuous sheet of cells.