The production of biodiesel from fast growing and lipid containing marine microalgal species is sustainably and economically more promising than fresh water microalgae. However, like fresh water microalgae, the process is limited by numerous factors such as the generally dilute nature of microalgal cultures, and the small size of microalgae cells, except for multi-cellular species such as Spirulina. Current dewatering technologies are sufficiently able to separate microalgae from the culture media; however the economics of the process makes these technologies ineffective as they are all too expensive when compared with the low cost of biodiesel from other sources. Centrifugation, which is one of the current technologies, is highly energy intensive especially in a large-scale setting. Filtration techniques such as tangential flow filtration has the potential to be a low cost dewatering technique, however there is a cost issue associated with the replacement of clogged membranes. Flocculation is another commonly used dewatering technique that has the advantage of using less energy under optimum conditions. Thus process development for marine microalgae flocculation could be an essential step to revolutionize biodiesel production from microalgae. In this work, mixed cultures of marine microalgal species were obtained from semi-continuous laboratory reactors and flocculation was investigated using polyelectrolyte (polymer) flocculants. Cationic, anionic and non-ionic polyelectrolyte flocculants were tested using the standard jar stirrer test at varying pH and temperature. All three flocculant types displayed suitability for microalgae flocculation with the cationic polymer obtaining the highest flocculation efficiency of 89.9% at an optimum concentration of 4 mg/L. Focused beam reflectance measurements (FBRM) showed real time changes in microalgal flocs size during the flocculation process. This data is essential to understand the kinetics of microalgal flocs formation, to ensure the stability of the floc formation process, and to monitor and evaluate the performance of the flocculation process. (C) 2010 Elsevier B.V. All rights reserved.