Bacteroides intermedius includes two distinct groups of organisms that are phenotypically indistinguishable by conventional methods. These two groups are represented by the type strain of the species ATCC 25611T (B. intermedius type I) and by ATCC 33563 (B. intermedius type II). Members of each group can be distinguished from each other by analysis of the cellular protein composition by SDS-polyacrylamide gel electrophoresis (SDS-PAGE) and by DNA-DNA homology studies, because they share less than 40% homology. The purpose of this study was to prepare specific DNA probes for the two groups of Bacteroides intermedius and to test them against field isolates. Whole-cell DNA probes were prepared from B. intermedius types I and II and tested against 253 field strains of Bacteroides which had been identified by conventional phenotypic tests as B. intermedius. Of these, 170 (67%) hybridized with the B. intermedius type I DNA probe, 28 (11%) with the type II, and 23 (9%) failed to react with the B. intermedius probes but did hybridize with either B. melaninogenicus, B. loescheii, or B. corporis whole-cell DNA probes. The 32 (13%) remaining isolates failed to hybridize with any of the five Bacteroides probes or with probes to B. asaccharolyticus, B. buccae, B. buccalis, B. denticola, B. gingivalis, B. oralis, or B. oris. These data demonstrate the usefulness of whole-cell DNA probes for the identification of phenotypically similar or identical field isolates.