In an effort to search for an efficient and environmentally friendly harvesting method, a commercially available microbial flocculant poly (gamma-glutamic acid) (gamma-PGA) was used to harvest oleaginous microalgae. Conditions for flocculation of marine Chlorella vulgaris and freshwater Chlorella protothecoides were optimized by response surface methodology (RSM) and determined to be 22.03 mg L-1 gamma-PGA, 0.57 g L-1 biomass, and 11.56 g L-1 salinity, and 19.82 mg L-1 gamma-PGA and 0.60 g L-1 biomass, respectively. Application of the two optimized flocculation methods to Nannochloropsis oculata LICME 002, Phaeodactylum tricornutum, C. vulgaris LICME 001, and Botryococcus braunii LICME 003 gave no less than 90% flocculation efficiency and a concentration factor greater than 20. Micrographs of the harvested microalgal cells showed no damage to cell integrity, and hence no lipid loss during the process. The results show that flocculation with gamma-PGA is feasible for harvesting microalgae for biodiesel production. (C) 2012 Elsevier Ltd. All rights reserved.