Azospirillum brasilense ATCC 29145, which possesses a single polar flagellum and also lateral flagella, exhibited swarming on the surface of nutrient broth containing 0.75% agar or 0.5% carrageenan. Various agents such as p-nitrophenylglycerol or sodium deoxycholate inhibited the swarming. Based on mutants lacking either polar or lateral flagella, the lateral flagella were responsible for the swarming on solid media, whereas the polar flagellum was responsible for swimming motility in liquid media.