This paper investigates the extraction of microbial polymers (polysaccharides) from fermentation broths of Sinorhizobium meliloti M5N1CS using crossflow filtration through ceramic membranes of various pore sizes from 0.1 to 0.8 mu m. The duration of fermentation was set at 70 h in order to maximize the production of high molecular weight polysaccharides (average 2 x 10(5) Da). The 0.1 mu m membrane underwent rapid fouling and was found inadequate for this application. For the other membranes, the sieving coefficients decreased from 95% to about 20% in 90 min, at a slower rate than the permeate flux. The largest permeate and mass fluxes were obtained with the 0.5 mu m membrane (18.5 x 10(-6) ms(-1) and 20 x 10(-6) gm(-2) s(-1)). Increasing the fluid velocity from 3 to 6 ms(-1) increased both the permeate flux and sieving coefficients, while raising the transmembrane pressure from 50 kPa to 100 kPa increased the flux slightly but decreased the sieving coefficient. Polysaccharide extraction will be maximized by operating at high velocities and low transmembrane pressure (TMP) which may require cocurrent recirculation of the permeate. Experiments with cell-free solutions showed that the permeate flux is mostly limited by the bacterial layer deposited on the membrane while the presence of cells has a positive effect on the sieving coefficient. Irreversible fouling due to polymer adsorption on the membrane decreased with increasing pore size and velocity but increased strongly with TMP.