: Pseudomonas aeruginosa biofilm formation was increased by the addition of sucrose to the LB medium, whereas the addition of NaCl to a final similar osmolarity and the use of maltose instead of sucrose, were ineffective. In a previous study, we showed that the extracytoplasmic sigma factor SigX is activated in the presence of sucrose. The sucrose-mediated pellicle increase effect was abolished in a sigX mutant strain. Sucrose addition led to increase pel expression and cyclic-diguanylate (c-di-GMP) pool level production. Interestingly, these two phenotypes were strongly lowered in a sigX mutant. Since pel is not known as a SigX-target, we suspected SigX to be involved in the c-di-GMP production. Interestingly, we found that expression of the diguanylate cyclase PA4843 gene was increased in presence of sucrose, at least partly though SigX activity. Taken together, our study shows that sucrose itself rather than osmolarity favours the biofilm way of life of P. aeruginosa through the activation of SigX. This article is protected by copyright. All rights reserved.