An electron microscope study of normal and L-forms of a marine luminous bacterium was carried out. In addition to the features found normally in Gram-negative bacilli, observed in cells from solid medium, cells from actively luminescent liquid culture were found to possess certain peculiar features. The most notable of these were tetrads, cells in which ingrowth of the cytoplasmic membrane causes the contents to be divided up into four protoplasts within the cell wall. Small cytoplasmic spherules which arise from the cytoplasmic membrane were also seen to occur. The cell walls which were three-layered and typical of Gram-negative bacteria often had blebs on the surface. The cytoplasmic membrane, typical of a 7.5 nm unit biological membrane in appearance in normal cells seemed to be composed of a number of laminations in some L-form cells. These laminations consisting of alternate dark and light bands may consist of numbers of unit membranes with adjacent protein layers fused. The occurrence of L-form cells was shown to be associated with luminescence and aeration in liquid sea-water medium.