The Legionnaires' disease (LD) bacterium, for which the name Legionella pneumophila has been proposed (1), is a small gram-negative microorganism that varies considerably in length depending upon its environment (2). Early investigators did not describe motility or the presence of flagella. While examining direct fluorescent antibody-stained smears of pure cultures of LD bacterium, we have observed fluorescent polar flagella on two stock strains (Bellingham 1 and Bloomington 2). We confirmed these observations by examining smears stained with either a silver-plating stain for flagella (3) or a simplified Leifson flagella stain (4). This observation prompted us to examine other strains of