Protease production byBacteroides fragilis ATCC 25285 was determined in batch and continuous cultures. During exponential growth in batch culture, the majority of proteolysis was cell associated. However, as the bacteria reached stationary phase, most of the intracellular proteases were released into the culture medium. Measurements of alkaline phosphatase and β-galactosidase, which are respectively periplasmic and cytoplasmic marker enzymes inB. fragilis, showed that secretion of proteases in the stationary phase was a discrete event and was not associated with a general release of cytoplasmic contents. When the bacterium was grown in continuous culture, cell-associated protease activity increased concomitantly with dilution rate (D=0.03–0.23/h). The ratio of intracellular to whole cell protease activity also increased with growth rate (1∶1 at D=0.03/h; 1∶1.7 at D=0.23/h). Extracellular protease activity was detected only in trace amounts in continuous cultures at the lowest dilution rate. Determinations of the distribution of extracellular protease activity in batch culture after 48 h incubation showed that the majority of proteolysis (ca. 90%) was soluble. Nevertheless, a proportion was associated with particulate fractions, which had high specific activities.