Nine strains of oral Fusobacterium were examined for their ability to coaggregate in vitro with four strains of the oral yeast. Candida albicans. All of the Fusobacterium nucleatum strains and Fusobacterium periodontium and Fusobacterium sulci coaggregated to various degrees with all of the Candida strains. Fusobacterium alocis, Fusobacterium mortiferum and Fusobactrium simiae strains did not coaggregate with any of the Candida strains. Exposure of the coaggregating Fusobacterium strains but not the Candida strains to heat, trypsin, and proteinase K eliminated coaggregation. Amphotericin B or trichodermin treatment of the yeast species had no effect. The reactions were inhibited by addition of 0.1 M mannose, glucosamine and alpha-methyl mannoside. All coaggregating pairs were disaggregated by the addition of sodium dodecyl sulfate, but nonionic detergents had no effect. The addition of 2.0 M urea completely reversed coaggregation. Candida strains were sensitive to periodate oxidation, whereas the Fusobacterium strains were stable to this treatment. All coaggregations occurred in the presence of saliva and appeared stronger than in buffer. These data suggest that the coaggregations involve either a protein or glycoprotein on the Fusobacterium surface, which may interact with carbohydrates or carbohydrate-containing molecules on the surface of the Candida. These observations expand the known range of intergeneric coaggregations occurring between human oral microbes and indicate that coaggregation of C. albicans and Fusobacterium species may be an important factor in oral colonization by this yeast. The authors believe this to be the first description of coaggregation concerning a carbohydrate component on the yeast cell and a protein component on the oral bacterial cell.