Physiological studies were conducted in an attempt to elucidate the mechanism of inhibition of Bacillus pumilus by Propionibacterium freudenreichii subsp. shermanii. Inhibition of B. pumilus by P. shermanii occurred in media supplemented with 1% glucose, indicating that glucose utilization by the latter bacterium was not responsible for growth inhibition of the former bacterium. The medium pH in which P. shermanii inhibited the growth of B. pumilus was 4.3. Propionic acid was positively identified in the culture medium in which B. pumilus was inhibited by P. shermanii. The presence of propionic acid and a low medium pH may account for the inhibition of B. pumilus by P. shermanii. Sodium lactate concentrations of 0.8-1.0% were essential for the continuous growth of and propionic acid production by P. shermanii. Thus, use of P. shermanii to inhibit B. pumilus in foods would likely require a lactate source.