A Gram-positive soil bacterium Bacillus subtilis is often exposed to osmolarity changes due to alternate flooding and drying of soil. Osmotic stress may damage bacterial cells and slow their growth, but it also induces an adaptive response that helps bacteria survive osmolarity changes. B. subtilis monitors the population growth by the ComQXPA quorum sensing (QS) system that also affects biofilm development. However, it is not known whether and how this quorum sensing system affects biofilm development at increased salinity. To address this question we monitored morphology, growth, and biomass of the wild type and signal mutants in/on MSgg medium with different NaCl concentrations (0, 2, 4, 6 % (w/V)). We used different quorum sensing and quorum sensing response mutants: ∆comQ, ∆comP, ∆comA, ∆srfA, and ∆comQ ∆srfA. We also monitored viscosity of the wild type and mutants comQ, srfA and comQ srfA. Moreover, we evaluated epsA expression in the wild type and comQ mutant. The results indicate that increased salinity negatively affects pellicle development and colony growth of the wild type. It also decreases biomass, viscosity and wrinkling of both biofilm types, a colony and a pellicle. ComQ and comP mutants develop biofilms faster than the wild type at all tested NaCl concentrations, whereas the development of biofilms of srfA, comQ srfA and comA mutants is similar or faster only at the increased salinity. Biofilm of mutants are more viscous than the wild type at all tested NaCl concentrations. However, the dynamics of epsA expression does not explain the increased biofilm viscosity. Based on the results we conclude that the ComQXPA quorum sensing system downmodulates the biofilm development of B. subtilis and it does so at all tested salinities.