A pH-controlled continuous lactic acid fermentation of whey permeate supplemented with yeast extract was carried out with Lactobacillus helveticus entrapped in kappa-carrageenan-locust bean gum gel beads at various dilution rates and pH set points. A kinetic study of the immobilized cell continuous fermentation in a 1-l stirred-tank reactor containing 33% (v/v) gel beads was carried out with an original experimental device, consisting of a 110-ml second-stage free-cell fermenter. This second stage was assumed to reflect the physiological behavior of the free-cell phase in the immobilized-cell bioreactor. A maximum lactic acid production rate of 28.5 gl(-1) h(-1) was measured at pH 5.5 and dilution rate 1.21 h(-1) for the immobilized-cell bioreactor. The overall specific growth rate and the specific lactic acid production rate of entrapped cells were 12-18% those of free cells in the reactor. This low activity may result from the presence of high inhibition in the central part of the beads as a result of substrate, product, and pH gradients. However, entrapped cells were responsible for 75-85% of the lactic acid and biomass productions within the immobilised cell fermenter at pH 4.7-6.3 and close to 90% at pH 4.3.