The occurrence and surface propertiesofPrevotella intermediaandP. nigrescensin healthy sites and in periodontic and endodontic infections were studied among 73 strains, tentatively identified asP. intermedia. Fifteen strains were from necrotic root canal infections, 41 were from periodontal samples, and 17 isolates were obtained from healthy gingival sites. Identification of isolates as eitherP. intermediaorP. nigrescenswas based on differences in malate and glutamate dehydrogenase electrophoretic mobilities which allowed unambiguous separation ofP. intermediaandP. nigrescens. The majority of strains from periodontal samples wereP. intermedia(29 of 41 strains). In endodontic samples only 4 out of 15 isolates wereP. intermedia, while all except 1 of 17 strains from healthy gingival sites were identified asP. nigrescens. SDS‐PAGE of whole cell proteins revealed 31 and 38 kDa proteins inP. nigrescenswhich were not detected inP. intermedia. Surface biotinylation of cells, followed by Western blotting and detection by alkaline phosphatase conjugated extravidin, showed strong staining of the 31 kDa protein inP. nigrescensindicating that this protein is located on the surface of the cell. Corresponding staining was not seen inP. intermedia. Fimbria‐like projections were observed using electron microscopy of negatively‐stained cells ofP. nigrescens. The results show thatP. intermediaandP. nigrescensmay have different site specificities and surface properties and thus emphasize the need for accurate identification of these two species for the evaluation of their role in the pathogenesis of oral infections.J Periodontol 1994;65:56–61.