Aim of the present work is quantifying the mechanical stress generated by some major process equipment used in massive microalgae culturing plants (centrifugal and air-lift pumps, and nozzles) and highlighting its effects on the microalgal population. Two microalgal species were used as test cases: Chlorella vulgaris (unicellular) and Scenedesmus dimorphus 1237 (colonial). The evaluation of the shear effect on algal growth was carried out through measurement of absorbance, photosynthetic activity (oxygen evolution) and variable chlorophyll fluorescence. Cell aggregate development/breakage was effectuated by visual inspection and light scattering. The use of centrifugal Pumps for culture recycling strongly affected the growth of C. vulgaris, while nozzles effects were confined to aggregate breakage of S. dimorphus. The analysis of experimental data is supported by the consideration of hydrodynamic stress calculated by: shear rate, shear stress, stress volumes/times, energy dissipation rates, and turbulence microscale size. (C) 2011 Elsevier Ltd. All rights reserved.