Although the molecular size distribution of dissolved organic matter (DOM) can affect the performance of biological wastewater treatment systems, limited information is available on the macromolecule breakdown in these reactors. In this study, we examined the degradation of a single, well-defined polysaccharide (dextran; 70,000 average molecular weight) in axenic culture using a dextran-degrading bacterium isolated from wastewater (CDC Group IIi). In suspended batch and continuous cultures fed dextran as a sole carbon source, there was an accumulation of both intermediate (<10,000 amu) and small (<1000 amu) molecular weight carbohydrates in solution. In batch culture, the maximum increase in concentration of intermediate-sized carbohydrates was 14% (17 mg l -1) of the initial dextran concentration. In continuous culture, 54% of the effluent polysaccharides were <10,000 amu although the influent contained less than 6% of this size fraction. This represents an overall net production in the small and intermediate sized carbohydrate concentrations in the chemostat of 143 mg l -1 (26%) and 116 mg l -1 (21%) under conditions where only 6% (34 mg l -1) of carbohydrates entering the chemostat were completely removed. These results indicate that polysaccarides can be released back into solution before being completely utilized by suspended microbes, producing large transformations in size distributions with only minimal changes in total carbohydrate concentrations.