The effect of diln. rate on the prodn. of lactic acid from whey permeate by L. helveticus was investigated. In the 1st chemostat of a 2-stage system, total conversion (98.1%) and max. lactic acid concn. (43.7 g/L) were obtained at a diln. rate (D1tot) of 0.06/h. Max. volumetric productivities of lactic acid (8.27 g/L-h) and biomass (1.90 g/L-h) occurred at D1tot of 0.40/h. The fraction of L-lactate in the product increased with diln. rate and reached a max. of 66% at the same diln. rate. The max. sp. growth rate (mmax) on this medium was 0.7/h. A YATP(max) value of 22.4 g dry wt./mol ATP and a maintenance coeff. of 8.0 mmol ATP/g dry wt.-h were detd. The 2nd stage, in series with the 1st, confirmed these results and also showed that the total residence time could be reduced by 50% compared with a single chemostat for the same nearly complete level of substrate conversion. [on SciFinder (R)]