A leaf spot and blight of SUGARBEET (Beta vulgaris L.) of unknown etiology was observed in several SUGARBEET production areas of Isfahan since 1993. Circular to irregular necrotic spots or blotches, less than one to several centimeters in diameter appear on leaves. The lesions progress inwards, expand and coalesce, giving the leaves a blighted appearance.A Gram-negative, aerobic, nonmotile, rod-shaped bacterium was consitently isolated from symptomatic leaves. Colonies on sucrose nutrient agar were nonmucoid, yellow, convex and smooth with entire margins. The strains were oxidase negative, nonfluorescent, and hydrolyzed casein, gelatin, esculin, starch and Tween-80. They were negative in tests for production of arginine dihydrolase, urease and lecithinase, reduction of nitrate and production of in,doleand acetoin. Some strains produced H2S from cysteine. Growth of the bacterium was inhibitedby 0.1% triphenyl tetrazolium chloride, 2.5% NaCI and 30% glucose. Fructose, glucose, cellobiose, trehalose, sucrose, maltose, melibiose, xylose, glycerol, starch, dextrin, amygdalin, arbutin, lactate, malate, D- and Lalanine and L- proline were among the carbon sources used for growth. No strain utilized adonitol, dulcitol, inositol, rhamnose or tartrates. Strains produced a xanthomonadin pigment.Pathogenicity of strains was confirmed through wound inoculation of young SUGARBEET plants.Based on the biochemical, physiological and nutritional charactristics, the strains isolated from, and proved pathogenic on SUGARBEET plants, were identified as Xanthomonas sp. The phenotypic features, electrophoretic profile of cell proteins and the type of symptoms produced by Xanthomonas sp. reported herein, are considerably different from those of Xanthomonas sp. betae (X campestris pv. betae) recorded in Brazil. Therefore, the blight of SUGARBEET occurring in Iran, appe.1rs to be a previously unreported disease and the incita"nt bacterium a new species or pathovar of Xanthomonas.