Bacteroides loescheii PK1295 serves as a coaggregation bridge between Streptococcus sanguis 34 and Actinomyces israelii PK14, two gram-positive oral bacteria that are otherwise unable to coaggregate. Whereas coaggregation with S. sanguis 34 is inhibited by lactose, no simple sugar was found that inhibited coaggregation with A. israelii PK14. Coaggregation-defective (Cog-) mutants of B. loescheii PK1295 were isolated for the purpose of identifying the surface components responsible for the interaction with each coaggregation partner. Selection for spontaneously occurring Cog- mutants gave rise to two phenotypic classes of mutants. Type I lost the ability to coaggregate with S. sanguis 34, whereas type II failed to coaggregate with either S. sanguis 34 or A. israelii PK14. Purified fimbriae from the parent agglutinated cells of both partners, and agglutination with S. sanguis 34 was inhibited by lactose. Denaturing polyacrylamide gel electrophoresis and immunoblot analysis demonstrated the presence of both a 75- and a 43-kilodalton (kDa) protein associated with parental fimbriae, but only a 43-kDa protein was seen with fimbriae prepared from the type I mutant. Neither polypeptide was found in similar preparations from the type II mutants. Our data suggest that coaggregation of B. loescheii PK1295 with both gram-positive partners is mediated by fimbria-associated proteins present on the surface of the gram-negative organism and that the 75- and 43-kDa polypeptides are responsible for the recognition of S. sanguis 34 and A. israelii PK14 cells, respectively.