The polysaccharides produced by Streptococcus thermophilus Rs and Sts in skimmed milk consist of D-Gal and L-Rha in a molar ratio of 5:2. Linkage analysis and 1D/2D NMR (1H and 13C) studies revealed that both polysaccharides have the same branched heptasaccharide repeating unit: [formula: see text] Remarkably, the two strains differ in their effects on the viscosity of stirred milk cultures. The milk culture of S. thermophilus Rs is non-ropy and affords 135 mg/L polysaccharide with an average molecular mass of 2.6 x 10(3) kDa. In contrast, the milk culture of S. thermophilus Sts is ropy and produces 127 mg/L polysaccharide with an average molecular mass of 3.7 x 10(3) kDa. Permeability measurements of non-stirred milk cultures of both strains suggest that both strains have a similar effect on the protein-polysaccharide network. Therefore, the only clear difference between both strains, which may cause the difference in ropiness of the milk cultures, is the difference in molecular mass of the polysaccharide.