Ten support materials including plastic-composite supports and six strains of propionibacteria were tested for their possible use in biofilm systems for enhanced production of propionic and acetic acid by fermentation. From screening experiments Propionibacterium thoenii strain P20 and fire bricks were chosen for further investigations;Propionibacterium thoenii P20 resists low-pH conditions, produces acid rapidly, forms luxuriant biofilms, and resists solvent inhibition better than other strains. Fire bricks are inexpensive, reusable, and compare favorably to commercial supports in ease of use and structural stability. A modified ilifesaveri shape for the individual fire brick particles was found to provide increased available surface area for biofilm formation and better flow patterns of the medium through and around supports. The attachment mechanism of bacterial cells to the supports was sought by measuring zeta potentials of both organisms and support materials, by hydrophobicity (MATH test) analysis, and by scanning electron microscopy (SEM) examination;To hold support materials and to provide better flow of medium through and around the supports, a novel stainless steel basket, BIOCAGE, was designed to fit into the fermenter. In sequential fed-batch fermentations, yields of propionic acid and acetic acid from substrate lactate have ranged from 63 to 79% and from 24 to 28%, respectively, with the higher yields obtained when biofilms were formed. Productivities for propionic and acetic acids have been relatively consistent at about 0.26 g/l/h and 0.095 g/l/h, respectively;Two immobilization methods, biofilm and calcium alginate entrapment, were compared in a mini reactor to determine the rates of substrate consumption and acid production per unit of the immobilized systems. Even though acid production with beads was higher than in the biofilm system, beads dissolved at the end of the first batch and started clogging outlet lines;Overall results indicate that P. thoenii P20 is a good biofilm former, and that biofilm fermentations can maintain high acid productivities even at low pH values.