The respiratory tract of patients with cystic fibrosis is a polymicrobial environment in which Pseudomonas aeruginosa is a dominant pathogen and oral commensal streptococci are emerging as cohabitants. Using a dual species biofilm model, Scoffield et al. showed that Streptococcus parasanguinis uses the P. aeruginosa-derived exopolysaccharide alginate to promote biofilm formation and colonization of the lungs, thereby interfering with P. aeruginosa pathogenesis. Although the streptococcal adhesin BapA1 is necessary for alginate-dependent biofilm formation in vitro, it is dispensable in vivo, in which fimbria-associated adhesion protein (Fap1) can also contribute to colonization. This study describes a new association between alginate and streptococcal adhesins, and reveals a new colonization mechanism for oral commensal streptococci in the cystic fibrosis lung environment.