Filtration characteristics of an asymmetric depth filter (nominal pore size: 0.2 mu m) were investigated by using suspensions of two bacteria, Lactobacillus plantarum and Bacillus amyloliquefaciens. In the filtration of L. plantarum suspensions, the permeation flux was 3-8 times higher with the depth filter than with a screen filter at 10-150 kPa. Apparent specific resistance of the cell layer in the depth filter reduced to about 1/10-1/100 of that of the filter cake formed on the screen filter. Scanning electron microscopy revealed that the trapped microbial cells were dispersed to a depth of 2/3 from the inlet side of the depth filter. In the filtration of B. amyloliquefaciens suspensions, the permeation flux of depth and screen filters was comparable at 10-50 kPa, and filter cakes formed on both filters. However, the permeation flux increased remarkably at 100-150 kPa with the depth filter but scarcely increased with the screen filter. The smallness of increase with the screen filter was due to the high compressibility of the microbial filter cake, while the remarkable increase with the depth filter was due to the penetration of the chain bacterium into the filter. The increase in transmembrane pressure was highly effective in the filtration of agglomerated chain bacteria such as B. amyloliquefaciens in trapping cells in the asymmetric depth filter.