A perfusion culture of Propionibacterium freudenreichii was coupled with a propionic acid removal system to realize high density cultivation at a low dilution rate. The propionic acid removal system consisted of a rotating ceramic membrane and a packed column of activated charcoal. The culture broth of P. frardenreichii was fed into the rotating ceramic membrane and then the resulting effluent was run through the packed column. After removing propionic acid which seriously inhibits the growth of cells, the permeate was recycled to the fermenter. In this cultivation system, a cell concentration of 157 g/dm(3) was attained at a dilution rate of less than 0.034 vvh, and propionic and acetic acids were recovered as by-products. The efficiencies of cell mass and product formation based on the amount of glucose supplied mere improved compared to those in the case of a conventional perfusion culture without a propionic acid removal system. The proposed perfusion culture system was effective in reducing the amount of discharged waste.