We studied the cell-envelope structure of the Legionnaires' disease (LD) bacterium by electron microscopy and biochemical assays. There were apparent differences in cell structure by electron microscopy using two different prefixation methods. Organisms prefixed with gluteraldehyde had a single surrounding membrane. The typical two-membrane structure of gram-negative bacteria, however, was observed after prefixation with a combination of gluteraldehyde, formalin, and creosol. The cell wall (peptidoglycan) was seen in electron micrographs of plasmolyzed bacteria and in papaindigested cells. Both cytoplasmic and outer membranes were separated by differential centrifugation of spheroplast sonicates followed by sucrose density gradient ultracentrifugation. We identified each membrane by characteristic enzyme activity (cytoplasmic membrane) and 2-keto-3-deoxyoctonate content (outer membrane).