The structures of the extracellular polysaccharides elaborated by Streptococcus mutans 03-5 and S. mutans OMZ 51 were investigated. These were fractionated into soluble fractions (GS-5B and OMZ 51B polysaccharides) and lesser-soluble fractions (GS-5A and OMZ 51A polysaccharides). GS-5B polysaccharide was essentially an a linked polyglucan in which 17% of the glucose units were linked through positions 1, 3 and 6 and formed branch points. 60% of the molecule comprised a single -linked glucose chain and the minimum average external chain length was 2. The polysaccharide was poly-dispersed M - 49)000 with a structurally similar low molecularweight fraction = 10,000. The following average repeating unit was proposed. GS-5A polysaccharide contained approximately 35% fructose and 40% of glucose. The polyglucan was linked through positions 1,3 and 6 and formed branch points. Approximately 3% of the glucose units were linked, through positions 1 and 3 only. An average repeating unit was proposed.