Six clinical isolates of the Legionnaires' disease bacterium have been isolated from patients in two Michigan hospitals. These organisms were recovered and cultivated in vitro and the strains named by city of recovery and order of isolation Detroit 1 to 4 and Flint 1 and 2. All strains, studied by scanning and transmission electron microscopy preparations from 7-day growth on agar, showed similar characteristics. By scanning electron microscopy the cells appeared bacillary, straight, and slightly curved, and spindle shapes and forms with rounded ends were observed. Detroit 2 and 3 and Flint 1 strains showed cells with ends having reduced diameters, the cells resembling an old-fashioned rolling pin. Cell widths ranged from 0.29 to 0.39 micron, and average lengths varied from 1.60 to 2.34 microns. Filamentous forms as long as 9.8 microns were observed. Surfaces appeared quite wrinkled compared to that of an Escherichia coli control organism. As seen by transmission electron microscopy, fine structure included a multilayered cell wall characteristic of gram-negative bacteria that was irregular, reflecting wrinkling; plasma membrane; and periplasmic space. Distinct nucleoid, intracytoplasmic ribosomes and rare vacuoles were also seen. No evidence of a spore or an adversity-resistant body was observed.