The effect of pH on the growth and proteolytic activity of the type strain and fresh isolates of Porphyromonas gingivalis and Bacteroides intermedius was investigated. B. intermedius strains grew with and without glucose at a pH as low as 5.0. These bacteria grew almost as well as Streptococcus mutans at pH 5.0 and better than Actinomyces viscosus at pH 5.5 and 5.0. Some B. intermedius strains raised the culture pH when grown at a low pH without glucose. In contrast, P. gingivalis strains grew only at pH 6.5 to 7.0. The P. gingivalis strains had proteolytic activities against azocoll, azocasein, and azoalbumin, while the B. intermedius strains degraded azocasein and azoalbumin, but not azocoll. B. intermedius showed maximum proteolytic activity at pH 7.0, and high activity over a wide pH range. In contrast, the optimum pH of proteolytic activity in P. gingivalis was pH 7.5 to 8.0. The P. gingivalis activities were more sensitive than those of B. intermedius to low pH. The capacity of B. intermedius to degrade proteins to more readily metabolizable substrates at low pH might explain the growth of this bacterium in an acidic environment. These differences between B. intermedius and P. gingivalis could explain their capacity to survive at different sites in the oral cavity and indicate how B. intermedius might positively influence the growth of P. gingivalis in subgingival plaque.