Five strains of Bacteroides fragilis isolated from human feces were assayed for their inhibitory activities against Shigella flexneri in vitro. Inhibition was not detected when Shigella and the Bacteroides strains were simultaneously inoculated into a defined liquid medium containing glucose. Inhibition was apparent only when Shigella was inoculated into established Bacteroides cultures. In exponential-phase cultures, the Shigella growth rate was diminished and logarithmic growth was prematurely interrupted. Shigella failed to multiply at all in stationary-phase Bacteroides cultures. An analysis of the stationary-phase cultures revealed that acetic and propionic acids were present in sufficiently high concentrations and the pH of the culture medium was sufficiently low to account for the inhibition of Shigella growth. In glucose-free Bacteroides cultures, Shigella multiplied almost as well as in control cultures, despite the presence of high concentrations of volatile fatty acids. At the high pH levels present in the glucose-free cultures, the acids were not toxic for Shigella. These results reaffirm the importance of the relationship between volatile fatty acids and pH in the inhibition of Shigella growth.