Cells of the propionate-tolerant strain Propionibacterium acidipropionici P200910, immobilized in calcium alginate beads, were tested for propionic and acetic acid production both in a semidefined laboratory medium and in corn steep liquor in batch, fed-batch, and continuous fermentation. Cell density was about 9.8 x 10(9) cells/g (wet weight) of beads, and beads were added to the medium at 0.1 g (wet weight) beads/ml. Beads could be reused for several consecutive batch fermentations; propionic acid production in the tenth cycle was about 50%-70% of that in the first cycle. In batch culture complete substrate consumption (glucose in semidefined medium, lactate in corn steep liquor) and maximum acid production were seen within 36 h, and acid yields from the substrate were higher than in free-cell fermentations. Fed-batch fermentations were incubated up to 250 h. Maximum propionic acid concentrations obtained were 45.6 g/l in corn steep liquor and 57 g/l in semidefined medium; this is the highest concentration achieved to date in our laboratory. Maxi mum acetic acid concentrations were 17 g/l and 12 g/l, respectively. In continuous fermentation of semidefined medium, dilution rates up to 0.31 h(-1) could be used, which gave higher volumetric productivities (0.96 g l(-1) h(-1) for propionic acid and 0.26 g l(-1) h(-1) for acetic acid) than we have obtained with free cells. Corn steep liquor shows promise as an inexpensive medium for production of both acids by immobilized cells of propionibacteria.