Propionic and isobutyric acids were found to be the major inhibitory metabolites for carboxymethyl(CM)-cellulase in high density culture of a recombinant Bacillus megaterium pCK108. These organic acids accumulated significantly at the glucose feed rate of 12g/hr. They inhibited CM-cellulase production by about 80% even at 20mM and 10mM of propionic and isobutyric acids, respectively.