The major cell wall polymer of Streptomyces sp. VKM Ac-2125, the causative agent of potato scab, is galactomannan with the repeating unit of the following structure: [carbohydrate structure in text] The polysaccharide with such a structure is found in the bacterial cell wall for the first time. The cell wall also contains small amount of a teichoic acid of the poly(glycerol phosphate) type and 3-deoxy-D-glycero-D-galacto-non-2-ulopyranosonic acid.