SUMMARYThe cultural and physiological characteristics of some Xanthomonas spp. that are typical of the genus were compared with two atypical species, Xanthomonas uredovorus and X . stewartii. Electron microscopic studies of X . uredovorus, prepared by two methods, show that this organism possesses peritrichous flagella. This fact, together with evidence of the fermentative metabolism of carbohydrates, should exclude this bacterium from the genus Xanthomonas. The interpretation of electron micrographs is discussed, and the systematic position of X . uredovorus and X . stewartii.