Two members of the Bacteroidesfragilis group, B. ovatus and B. thetaiotaomicron, are difficult to distinguish by biochemical methods. They are currently identified on the basis of their variable ability to ferment salicin. We studied a method of identification for these two species by using cell lysis by bacteriophages. A total of 38 bacteriophages were used to distinguish the two species. Identification by bacteriophages was compared with species identification by prereduced anaerobically sterilized biochemical testing with salicin as the differentiating test. A total of 215 clinical isolates biochemically identified as B. ovatus or B. thetaiotaomicron were tested. A total of 100% of the strains identified as B. ovatus by bacteriophages produced strong acid in salicin (pH s 5.4). However, 40% of the strains identified as B. thetaiotaomicron by bacteriophages also produced strong acid in salicin, and an additional 39% produced weak acid (pH 5.5 to 5.7). This study demonstrates that salicin fermentation is an inadequate test for the differentiation of B. ovatus and B. thetaiotaomicron.