The effects of the substrate conditions on the volumetric productivity of Lactobacillus helveticus at different cell densities up to 60 g l(-1) in a continuous stirred-tank reactor with microfiltration to retain the biomass were investigated. At low dilution rates, D, the steady-state volumetric productivity, r(p), gradually increased to a maximum at D = 1.2-1.5 h(-1), because of reduced product inhibition. At higher D values, r(p) unexpectedly decreased, although the substrate conditions further improved. The maxima of r(p) at different cell densities coincided with a critical specific substrate utilization rate beyond which the cell metabolism seems to be controlled through a catabolic modulator factor, and r(p) decreases.