Cross flow filtration（CFF） has been successfully employed as a means of concentrating microbial cells and proteins. While the CFF of proteins has been investigated thoroughly and could be explained in terms of the gel layer theory, the investigations on the microbial cells have been limited to the applications like concentration. In this study we aimed at studying the characteristics of CFF of microbial cells more systematically. Unlike in proteins, CFF of microbial cells could not be explained in terms of the gel layer theory. Fluxes were independent of the pressure difference which exceeded a certain level, but they increased with the flowrate in the hollow fiber and were proportional to U^0.5 and U^0.8 in the cases of E. coli and yeast respectively. Gel layer theory employing the diffusivity based on the Stokes-Einstein equation was applied to the system. But the predictions were far apart from the experimental data. Thus we suggested that the causes of the discrepancies between the theory and the experiment might be due to the tubular pinch effect or other phenomena such as gel layer slippage.