Flocculation induced by pH increase for harvesting microalgae and reuse of flocculated medium were evaluated. Increasing the medium pH value induced the highest flocculation efficiency of up to 90% for freshwater microalgae (Chlorella vulgaris, Scenedesmus sp., Chlorococcum sp.) with low/medium biomass concentrations and marine microalgae (Nannochloropsis oculata, Phaeodactylum tricornutum). The mechanism may be explained that Mg2+ in the growth medium hydrolyzed to form magnesium hydroxide precipitate, which coagulated microalgal cells by sweeping flocculation and charge neutralization. Additionally, this study revealed that the microalgal biomass concentrations and released polysaccharide (RPS) from microalgae could influence the flocculation efficiencies. Furthermore, neutralizing pH and then supplementing nutrients allowed the flocculated medium to maintain an approximate growth yield to that of the fresh medium in algal cultivation. These results suggest that the method presented here is effective, and allows the reuse of the flocculated medium, thereby contributing to the economic production from algae to biodiesel. (C) 2012 Elsevier Ltd. All rights reserved.