The architecture of microalgae biofilms has been poorly investigated, in particular with respect toshear stress, which is a crucial factor in biofilm-based reactor design and operation. To investigatehow microalgae biofilms respond to different hydrodynamic regimes, the architecture and cohesionof Chlorella vulgaris biofilms were studied in flow-cells at three shear stress: 1.0, 6.5 and 11.0 mPa.Biofilm physical properties and architecture dynamics were monitored using a set of microscopictechniques such as, fluorescence recovery after photobleaching (FRAP) and particle tracking. At lowshear, biofilms cohesion was heterogeneous resulting in a strong basal (close to the substrate) layerand in more loose superficial ones. Higher shear (11.0 mPa) significantly increased the cohesion ofthe biofilms allowing them to grow thicker and to produce more biomass, likely due to a biologicalresponse to resist the shear stress. Interestingly, an acclimation strategy seemed also to occurwhich allowed the biofilms to preserve their growth rate at the different hydrodynamic regimes. Ourresults are in accordance with those previously reported for bacteria biofilms, revealing some generalphysical/mechanical rules that govern microalgae life on substrates. These results may bring newinsights about how to improve productivity and stability of microalgae biofilm-based systems.