The structures of extracellular polysaccharides produced by the plant pathogensAgrobacterium tumefaciens(2 strains),A.rhizogenes,A.radiobacter,A.ruhi,A.gypsophilae, andA.pseudotsugaewere investigated. Polymeric material was formed in good yield from all organisms exceptA.gypsophilae. Periodate oxidations of the polysaccharides and their sodium borohydride reduction products indicated linear 1,2-glucopyranose structures forA.tumefaciens(2 strains),A.rhizogenes,A.radiobacter, andA.rubipolymers. Optical rotational considerations led to the assignment of β-configurations. The linear structure for theA.radiobacterpolysaccharide was confirmed by the isolation, after partial acid hydrolysis, of five 1,2-linked oligosaccharides with chain lengths of two to six. Periodate and chromatographic data indicated that the polysaccharide fromA.pseudotsugaecontained glucose and galactose and 1,6- as well as 1,2-glycopyranose links. It is noteworthy that the polysaccharides from the genusAgrobacteriurmare shown to be mainly 1,2-β-glucans, a structure not yet found in other genera.Treatment of linear 1,2-β-glucans fromA.tumefaciens,A.radiobacter, andA.rhizogeneswith sodium hydroxide – dimethyl sulphate, followed by silver oxide in methyl iodide dimethyl formamide, did not cause full methylation. Some of the 3-hydroxyl groups were not substituted and possible reasons for this unreactivity are discussed.