The performance of a two-stage system with cell recycling and fresh feed at each stage is studied numerically for continuous acetic acid production. In this system, both filtrate and bleed broth from the first stage are supplied to the second fermenter. At high substrate conversions, this configuration is found to provide higher acetic acid productivities than either a previous configuration where only the first-stage bleed broth is fed to the second stage or a single recycle chemostat at the same bleed ratios. (c) 2005 Society of Chemical Industry.