In the lung mucus of cystic fibrosis (CF) patients, P. aeruginosa forms matrix-enclosed microcolonies which cause chronic infections representing the major cause of mortality in CF-patients. The lung mucus of CF-patients is probably characterized by low fluid-shear as the main shear-causing factor, i.e. mucociliary clearance, is absent. In this study, the influence of fluid-shear on the growth behavior of P. aeruginosa PA01 was investigated using a low-shear suspension culture device, the RWV. Cultivation in low-shear induced a self-aggregating phenotype of P. aeruginosa PA01, resulting intheformationofbiofilms in suspension similar to what has been described in CF-mucus. The addition of a ceramic bead to the culture medium in the RWV created a higher-shear condition which led to the formation of surface-attached rather than suspensionbiofilms. In low-shear cultureconditions, a significant increase of the rhl N-butanoyl-L-homoserine lactone directed quorum sensing (QS) system and the psl-polysaccharide synthetic locus was demonstrated using gene expression analysis. Accordingly, the low-shear condition induced a higher production of rhamnolipids, which is controlled by the C4-HSL QS-system and is known toplay a role in CF-lungpathology. These results indicate that fluid-shear has an impact on the growth phenotype of P. aeruginosa which might play a role in CF-lung infections caused bythis bacterium.