The feasibility of continuous production of proteins in chemostat cultures of Bacillus subtilis was investigated. An expression system consisting of the bacterium B. subtilis BR151 carrying plasmid p602/19 was used. The plasmid contains the cat (chloramphenicol acetyltransferase) gene downstream of a strong vegetative T5 promoter. It was found that, at a dilution rate of 0.2 h(-1), production of relatively high levels of CAT protein (about 4% of cellular protein) can be sustained. But, experiments at a higher dilution rate of 0.4 h(-1) were unproductive because of high acid formation and washout. Combination of low cell yield, which results from excessive acid formation, and low dilution rate led to a low volumetric CAT productivity. Our recent work with the nonrecombinant cells, has demonstrated that uptake of small amounts of citrate significantly reduces or entirely eliminates the acid formation. This superior performance in the presence of citrate was hypothesized, based on strong experimental evidence, to be the result of a reduction in glycolysis flux through a sequence of events leading to a reduction in pyruvate kinase and phosphofructokinase activities, the regulatory enzymes of glycolysis. In this study, it is demonstrated that cofeeding of glucose and citrate substantially reduces the organic acid formation and significantly increases the recombinant culture productivity. The combination of high specific CAT activity and cell density resulted in a total of six- to tenfold higher culture productivity when citrate and glucose were cometabolized than when glucose was the only carbon source.