Pseudomonas aeruginosa is a Gram-negative bacterium associated with nosocomial infections and cystic fibrosis. Chronic bacterial infections are increasingly associated with the biofilm lifestyle in which microcolonies are embedded in an extracellular matrix. Screening procedures for identifying biofilm-deficient strains have allowed the characterization of several key determinants involved in this process. Biofilm-deficient P. aeruginosa PAK strains affected in a seven-gene cluster called pel were characterized. The pel genes encode proteins with similarity to components involved in polysaccharide biogenesis, of which PelF is a putative glycosyltransferase. PelG was also identified as a putative component of the polysaccharide transporter (PST) family. The pel genes were previously identified in the P. aeruginosa PA14 strain as required for the production of a glucose-rich matrix material involved in the formation of a thick pellicle and resistant biofilm. However, in PA14, the pel mutants have no clear phenotype in the initiation phase of attachment. It was shown that pel mutations in the PAK strain had little influence on biofilm initiation but, as in PA14, appeared to generate the least robust and mature biofilms. Strikingly, by constructing pel mutants in a non-piliated P. aeruginosa PAK strain, an unexpected effect of the pel mutation in the early phase of biofilm formation was discovered, since it was observed that these mutants were severely defective in the attachment process on solid surfaces. The pel gene cluster is conserved in other Gram-negative bacteria, and mutation in a Ralstonia solanacearum pelG homologue, ragG, led to an adherence defect.