In the fermentation of Escherichia coli, a ceramic membrane filter was used in conjunction with a fermenter for cell-recycle operation. The metabolic behaviour, especially the acetic acid production and the specific growth rate, under different concentrations of dissolved O-2 was investigated. An unstructured model was constructed, and it was shown that the observed lower cell density under low dissolved O-2 concentrations was accompanied by large acetic aicd production. However, the concentration of acetic acid was not high enough to cause the observed growth inhibition, and inhibition by other metabolites under low O-2 environment is suspected.