Pseudomonas aeruginosa galactose- and fucose-binding lectins (PA-IL and PA-IIL) contribute to the virulence of this pathogenic bacterium, which is a major cause of morbidity and mortality in cystic fibrosis patients. The crystal structure of PA-IIL in complex with fucose reveals a tetrameric structure. Each monomer displays a nine-stranded, antiparallel b-sandwich arrangement and contains two close calcium cations that mediate the binding of fucose in a recognition mode unique among carbohydrate-protein interactions. Experimental binding studies, together with theoretical docking of fucose-containing oligosaccharides, are consistent with the assumption that antigens of the Lewis a (Le(a)) series may be the preferred ligands of this lectin. Precise knowledge of the lectin-binding site should allow a better design of new antibacterial-adhesion prophylactics.