A mixture of endo- and exoxylanases were used to produce a hemicellulose hydrolysate containing 29.6 g L-1 of xylose equivalents based on reducing sugars analysis. Propionibacterium acidipropionici was grown in a 2-l batch reactor in mineral salts medium containing 60% (v/v) hemicellulose hydrolysate, 5 g l(-1) proteose peptone, and 2.5 g l(-1) yeast extract. Propionic acid production was growth associated. The maximum specific growth rate and maximum specific propionic acid production rate were 0.1 and 0.23 g g(-1) h(-1), respectively. Growth and acid production were inhibited at propionic acid concentrations greater than 2 g l(-1). The final concentration of propionic acid attained was 18 g l(-1). This study demonstrates the feasibility of propionic acid production by P. acidipropionici on an enzymatically hydrolyzed hemicellulosic fraction of steam-exploded Populus tremuloides.