Cells of the genus Azospirillum possess a single polar flagellum when grown in broth; when grown on solid media they also possess numerous lateral flagella of shorter wavelength (0.7 µm vs. 1.2 µm) and thinner diameter (13.5 nm vs. 18 nm). The antigenic and functional differences between these two types of flagella were studied for A. brasilense ATCC 29145 (type strain). An indirect immunoperoxidase stain was used in conjunction with electron microscopy to demonstrate that an antigenic difference that exists between these two types of flagella in this strain. No evidence could be obtained that the polar flagellum was ensheathed as it is in members of the genus Vibrio. A. brasilense was found to swarm on nutrient broth medium solidified with 0.75% agar ("swarm medium" or SM), and also on medium solidified with carrageenan. Swarming also occurred on a dialysis membrane overlying SM, indicating that the nature of the solid surface may not be important. The effects of various physical and chemical factors on swarming were determined. A temperature of 30°C rather than the optimal growth temperature of 37°C was stimulatory to swarming. Certain chemical agents, p-nitrophenylglycerol, EDTA, sodium deoxycholate, sodium taurocholate, and Na₂SO₄, at appropriate concentrations could inhibit swarming without preventing growth. Based on the use of mutants lacking either the polar flagellum or the lateral flagella, the lateral flagella were found to be responsible for swarming, whereas the polar flagellum was responsible for free-swimming motility. / M.S.