Propionibacterium freudenreichii subsp. shermanii can ferment glucose and glycerol to propionic acid with acetic and succinic acids as two by-products. Propionic acid production from glucose was relatively fast (0.19 g/L h) but gave low product yield (similar to 0.39 g/g) and selectivity (P/A: similar to 2.6; P/S: similar to 4.8). In contrast, glycerol with a more reduced state gave a high propionic acid yield (similar to 0.65 g/g) and selectivity (P/A: similar to 31; P/S: similar to 11) but low productivity (0.11 g/L h). On the other hand, co-fermentation of glycerol and glucose at an appropriate mass ratio gave both a high yield (0.54-0.65 g/g) and productivity (0.18-0.23 g/L h) with high product selectivity (P/A: 14; P/S: 10). The carbon flux distributions in the co-fermentation as affected by the ratio of glycerol/glucose were investigated. Finally, co-fermentation with cassava bagasse hydrolysate and crude glycerol in a fibrous-bed bioreactor was demonstrated, providing an efficient way for economic production of bio-based propionic acid. (C) 2013 Elsevier Ltd. All rights reserved.