In order for the opportunistic Gram-negative pathogen Pseudomonas aeruginosa to cause an airway infection, the pathogen interacts with epithelial cells and the overlying mucous layer. We examined the contribution of the biofilm polysaccharide Psl to epithelial cell adherence and the impact of Psl on proinflammatory signaling by flagellin. Psl has been implicated in the initial attachment of P. aeruginosa to biotic and abiotic surfaces, but its direct role in pathogenesis has not been evaluated (L. Ma, K. D. Jackson, R. M. Landry, M. R. Parsek, and D. J. Wozniak, J. Bacteriol. 188:8213-8221, 2006). Using an NF-kappaB luciferase reporter system in the human epithelial cell line A549, we show that both Psl and flagellin are necessary for full activation of NF-kappaB and production of the interleukin 8 (IL-8) chemokine. We demonstrate that Psl does not directly stimulate NF-kappaB activity, but indirectly as a result of increasing contact between bacterial cells and epithelial cells, it facilitates flagellin-mediated proinflammatory signaling. We confirm differential adherence of Psl and/or flagellin mutants by scanning electron microscopy and identify Psl-dependent membrane structures that may participate in adherence. Although we hypothesized that Psl would protect P. aeruginosa from recognition by the epithelial cell line A549, we instead observed a positive role for Psl in flagellin-mediated NF-kappaB activation, likely as a result of increasing contact between bacterial cells and epithelial cells.