Bacteroides fragilis is the anaerobe most frequently isolated from human infections. Strains of this species are not easily distinguished by phenotypic tests. It is important to make this distinction because virulence may vary between strains and because B. fragilis seems to be a heterogeneous species. The aim of this study was to assess the utility of randomly amplified polymorphic DNA (RAPD) analysis for differentiation of 46 strains of B. fragilis. Twenty-seven of the strains belonged to Johnson's DNA homology group I and 8 to group II, while 11 strains had not been assigned to any of these groups (NI group). The primers OPA16 and 18 were chosen among 30 primers tested for optimal RAPD analysis. OPA18 gave best discrimination, revealing a total of 15 genotypes while OPA16 gave 13. The gels obtained after RAPD analysis were evaluated with the Dendron computer-assisted program. Most strains showed similarity levels (SAB) within 70%. Strain clusters thus established were not always in agreement with DNA homology since strains from both homology groups fell in the same cluster. Similarly, strains of the NI group fell among the group I and II homology strains. RAPD was useful for differentiation of B. fragilis strains and thus probably suitable for epidemiological studies. On the other hand, DNA-DNA homology, comparing the entire genome of strains rather than a few random priming sites, would be more reliable for taxonomy. Computer-assisted gel analysis made it possible to objectively evaluate multiple banding patterns, thereby increasing the reliability of the RAPD analysis.