To produce propionic acid and vitamin B-12 from sucrose, the strain Propionibacterium acidipropionici NRRL B3569 was selected by screening a number of Propionibacterium strains. The nutrient composition and the fermentation conditions for this strain were optimized in continuous culture. The investigations show that within a concentration range of 30-170 g l(-1) of sucrose in the fermentation medium, no significant substrate inhibition occurred. For the production of propionic acid and vitamin B-12, concentrations of 1.5 mg FeSO4 . 7H(2)O g(-1) dry biomass, 0.75 mg cobalt ions g(-1) dry biomass, 0.3 mg 5,6-dimethylbenzimidazole g(-1) dry biomass, and 12 g yeast extract l(-1) were necessary additions to the sources of nitrogen, phosphate, and magnesium ions. The extra addition of up to 2.8 g betaine g(-1) dry biomass significantly increases the production of vitamin B-12. In the optimization of the pH value, temperature, and aeration, it was established that the conditions for propionic acid production and vitamin B,, production are different. Whereas the optimal production of propionic acid took place under completely anaerobic conditions with a pH value of 6.5 and a temperature of 37 degrees C, optimal vitamin B-12 production required a temperature of 40 degrees C and aerobic conditions (0.5 vvm aeration at 100 rpm) with a pH value of 6.5.