['We study the Azospirillum brasilense tolerance to water deficit and the dynamics of adaptive process at the level of the membrane.', 'Azospirillum brasilense was exposed to polyethylene glycol (PEG) growth and PEG shock. Tolerance, phospholipids and fatty acid (FA) composition and membrane fluidity were determined. Azospirillum brasilense was able to grow in the presence of PEG; however, its viability was reduced. Cells grown with PEG showed membrane fluidity similar to those grown without, the lipid composition was modified, increasing phosphatidylcholine and decreasing phosphatidylethanolamine amounts. The unsaturation FAs degree was reduced. The dynamics of the adaptive response revealed a decrease in fluidity 20\xa0min after the addition of PEG, indicating that the PEG has a fluidizing effect on the hydrophobic region of the cell membrane. Fluidity returned to initial values after 60\xa0min of PEG exposure.'] Is reorganization of Azospirillum brasilense cell membrane mediated by lipid composition adjustment to maintain optimal fluidity during water deficit?