A commercial strain of Streptococcus thermophilus possessing galactokinase activity was examined for the effect of lactose flow rate on the production and composition of polysaccharides through the early steps of biosynthesis. In all cases, lactose-grown cells did not release free galactose into the medium and produced polysaccharides containing galactose, glucose, mannose, uronic acids and minor amounts of hexosamines. In batch cultures with excess lactose present the cells converted nearly 80% of the carbon source to L-lactate and produced 2.4g 1-1 (eq. glucose) polysaccharides. However, when the carbon flow was set at 1.5 mM h -a, only 47% of the fermented sugar was converted to Llactate by the strain, which synthetized 22% more polysaccharides. As lactose became limiting, the level of some glycolytic enzymes and nucleotidyltransferases markedly decreased while phosphoglucomutase, phosphomannose isomerase and galactokinase activities were stimulated. The shift in the key enzyme ratios was reflected by major changes in polysaccharide distribution, which definitely altered in favour of galactose. Data suggested a diversion of lactose flow towards polysaccharide production at the expense of lactic acid and biomass formation, as well as a fine regulation of polymer distribution when the cell growth of S. thermophilus was limited by the carbon source feed rate.