The distribution of carbon in the metabolic network of a bacterial cell was estimated by a mass-balance-based intracellular flux computation method. It was applied to the growth phase of Corynebacterium melassecola, a glutamic acid producing bacterium, using experimental production yields of biomass, lactate and acetate measured during batch cultures on glucose, fructose, and various mixtures of both sugars. This flux computation method identifies the direction of the 86 reactions that ensure proper metabolic function during the growth phase of C. melassecola. Flux ratios allow comparison of calculated and relevant experimental yields. The results highlight the key influence of the biomass production yield Y-x-O2 on the overall distribution of carbon; the proportion of carbon drained in the pentose-P pathway fell from a value in the range of 54% to 47% on media containing glucose (Y-x-O2 = 1.75 to 1.56 g X/g O-2) to 37% on fructose medium (Y-x-O2 = 1.36 g X/g O-2). The highest maintenance requirement was calculated on fructose medium (J(m) = 290 mol ATP/100 mol fructose) which must be connected to a lower efficiency of cell multiplication observed on this substrate. Another important result was that the significant decreases in experimental values of production yields and rates observed on fructose medium which were related to the operation of the FBPase. In particular, it was estimated that, as long as the proportion of glucose in the carbon source remains above 22% (78% fructose), the operation of the FBPase is not necessary and the bacteria exhibit behavior similar to that observed on glucose alone; this result is consistent with experimental observations.