<p>Increasing evidence shows that proline accumulation may contribute to acclimation of microalgae to environmental stress such as excess salt or heavy metals. However, significant variability does exist among algal strains concerning the conditions that promote proline synthesis, as well as the intracellular levels of the amino acid that are reached and maintained under stress. Here we report on osmo-induced proline accumulation in the high biomass-yielding strain SAG 211-11p of <i>Chlorella vulgaris</i>. Algal cultures were grown mixotrophically and subjected to osmotic stress by the addition to the culture medium of increasing concentrations of either salts or non-ionic, non-permeant osmolytes. The intracellular concentration of proline was then measured in cells grown under conditions under which the growth rate had been found steadily reduced. A remarkable tolerance toward molar concentrations of NaCl was found, which was accompanied by an up to 24-fold increase of proline over basal levels. Results suggest the possibility of using marine or partially desalinated water sources for the cultivation of this strain.</p>