This work aimed to study the effects of chitosan on cell integrity and extracellular microcystins (MCs) of Microcystis aeruginosa cells during flocculation and flocs storage processes. The impacts of chitosan addition, flocculation stirring and flocs storage time were comprehensively detected to prevent or reduce cell lysis and MCs release. Response surface method (RSM) was applied to optimize the chitosan flocculation. Under chitosan concentration 7.31 mg/L and optimized mechanical conditions, 99% of M. aeruginosa cells were integrated removed. Furthermore, amounts of extracellular MCs were adsorbed by chitosan polymers in this process. With chitosan flocs protect, though cells showed some damage, extracellular MCs concentration in flocculated samples lower than background level within first 2 d. However, lots of MCs release was observed after 4 d which may result from chitosan degradation and cells lysis. Therefore, chitosan flocs should be treated within 2 d to prevent the adsorbed MCs releasing again. (C) 2013 Elsevier Ltd. All rights reserved.