A two-stage two-stream chemostat system and a two-stage two-stream immobilized upflow packed-bed reactor system were used for the study of lactic acid production by Lactobacillus casei subsp casei. A mixing ratio of D-12/D-2 = 0.5 (D = dilution rate) resulted in optimum production, making it possible to generate continuously a broth with high lactic acid concentration (48 g l(-1)) and with a lowered overall content of initial yeast extract (5 g l(-1)), half the concentration supplied in the one-step process. In the two-stage chemostat system, with the first stage at pH 5.5 and 37 degrees C and a second stage at pH 6.0, a temperature change from 40 degrees C to 45 degrees C in the second stage resulted in a 100% substrate consumption at an overall dilution rate of 0.05 h(-1). To increase the cell mass in the system, an adhesive strain of L. casei was used to inoculate two packed-bed reactors, which operated with two mixed feedstock streams at the optimal conditions found above. Lactic acid fermentation started after a lag period of cell growth over foam glass particles. No significant amount of free cells, compared with those adhering to the glass foam, was observed during continuous lactic acid production. The extreme values, 57.5 g l(-1) for lactic acid concentration and 9.72 g l(-1) h(-1) for the volumetric productivity, in upflow packed-bed reactors were higher than those obtained for free cells (48 g l(-1) and 2.42 g l(-1) h(-1)) respectively and the highest overall L(+)-lactic acid purity (96.8%) was obtained in the two-chemostat system as compared with the immobilized-cell reactors (93%).