Interbacterial adhesion between strains of Pseudomonas aeruginosa and strains of indigenous oral bacteria, both of which were isolated from the oral cavity of cystic fibrosis patients, was investigated by the phenomenon of the coaggregation reaction. A total of 22 strains of P. aeruginosa were isolated from the oral cavitiy of 17 patients and examined for their abilities to coaggregate with 5 strains each of Streptococcus sanguis, Streptococcus mitis, Actinomyces viscosus, and Actinomyces naeslundii. Coaggregation reactions were common between these oral bacteria and both the mucoid and nonmucoid variants of P. aeruginosa. All strains of P. aeruginosa were also able to agglutinate neuraminidase-treated or untreated human erythrocytes of blood types A, B, and O. Positive coaggregation reactions were further characterized by determining the effects of several sugars, and of heat and protease treatments of the bacteria. None of the coaggregation reactions were inhibited by 0.05 M lactose, galactose, glucose, fucose, or mannose. All coaggregation reactions were dependent upon heat- and protease-sensitive components of the Pseudomonas. Thus, the interbacterial adhesions between P. aeruginosa and the oral bacteria studied appears to involve adhesins on the Pseudomonas cell, which bind to complementary receptors, on the cell surfaces of oral bacteria. The apparent prevalence and diversity of interbacterial adhesions between P. aeruginosa strains originating from the oral cavity of cystic fibrosis patients and strains of the indigenous oral bacteria suggest that some of these reactions may affect the extent to which P. aeruginosa colonizes in the oral cavity of cystic fibrosis patients, and thereby, influence susceptibility of the host to infection.