Chemical mutagenesis with ethyl methanesulfonate followed by selective culturing was used to develop strains of Lactobacillus delbrueckii that tolerated increased lactic acid concentrations while continuously producing the acid. In stirred-tank-batch fermentations, mutant DP3 in 12% glucose, 3% yeast extract-mineral salt-oleic acid medium produced lactic acid more than four times faster than the wild type. Mutant DP3 produced 117 g/l lactic acid compared with 67 g/l for the wild type. Mutant DP3 exhibited faster specific growth rates, shorter lag phases, greater lactic acid fields, and better lactic acid tolerance than the wild type. Mutant DP3 was stable for over 1.5 years (stored freeze dried). Mutants with enhanced lactic acid-producing capacity were obtained each time the strain development procedure was employed;Combinations of lactobacilli and biofilm-forming bacteria were evaluated in continuous fermentations for lactic acid production using various supports. Twelve different bacteria, including species of Bacillus, Pseudomonas, Streptomyces, Thermoactinomyces, and Thermomonospora, were tested for biofilm-forming capabilities. Solid supports that were evaluated in either batch or continuous fermentations were pea gravels, porcelain berl saddles, 3M-macrolite ceramic spheres, and polypropylene (pp) mixed with 25% various agricultural materials (e.g. corn starch, oat hulls) and extruded to form chips. The best biofilm formation was observed with Pseudomonas fragi, Streptomyces viridosporus T7A, and Thermoactinomyces vulgaris. Biofilm formation on the pp-composite chips was detected at harvest by chip clumping and gram-staining. Lactic acid bacteria used in the fermentations were Lactobacillus amylophilus, Lactobacillus casei, and Lactobacillus delbrueckii mutant DP3. S. viridosporus T7A and L. casei on pp-composite chips were the best combination of those tested. In 25-ml continuous fermentations, biofilm reactors produced 10 to 14 g/l lactic acid at a flow rate of 0.24 ml/min, whereas 4.5 g/l of lactic acid was produced by the suspension culture. Furthermore, production rates that were two to five times faster than those of the suspension culture were observed for the pure- and mixed-culture bioreactors. Also, lactic acid concentrations and production rates were consistently higher in mixed-culture than in pure-culture fermentations on pp-composite chips.