Various amounts of propionic acid were added to the fermentation medium of Propionibacterium thoenii strain P20 to examine the effects of propionic acid on cell growth, product formation, and substrate consumption and yield. The fermentation kinetics obtained from the cell-immobilized reactor were free of mass transfer limitations. Such inherent kinetics were described by exponential models in terms of specific propionic acid productivity and the glucose consumption rate. As the propionic acid concentration in the fermenter increased from 2.77 to 30.41 g/l(-1), cell growth declined by two-thirds, specific propionic acid productivity and the glucose consumption rate also decreased from 0.059 to 0.015 g propionic acid g(-1) cell · h(-1), and 0.11 to 0.04 g glucose g(-1) cell · h(-1), respectively. The excess of propionic acid also altered the bacterial metabolism to produce more by-products such as acetic, lactic and succinic acids which resulted in the decrease of substrate yield on propionic acid from 0.52 to 0.41 g propionic acid g(-1) glucose. To overcome such effects, it is desirable to maintain the acid concentration as low as possible which can be achieved by extractive fermentation. At propionic acid concentrations below 3 g l(-1), a propionic acid volumetric productivity of 3.9 g l(-1) · h(-1) can be attained with 40% (w/v) bead load.