In this study, an original use of low-energy-consuming submerged MF/UF membranes was investigated for the total removal of toxic micro-algae (Alexandrium sp.) from seawater Experimentally, a non-toxic micro-alga (Heterocapsa triquetra) which has morphological and dimensional (15-20 mu m) similarities with Alexandrium sp. one of the smallest toxic micro-algae in seawater, was ultra- and micro-filtered A lab-scale membrane screening was operated on 10 kDa. 300 kDa and 0 2 mu m membranes by filtration of 30,000 cells mL(-1) of H triquetra suspension (equivalent to a major natural phytoplanktonic bloom) The most promising results were obtained with microfiltration membrane (0 2 mu m). the highest steady state permeate flux reached 29 L h(-1) m(-2) under 0 3 bar of transmembrane pressure after 180 min of filtration. More than 99%, 87% and 98% of micro-algae, Total Suspended Solid (TSS). and turbidity were eliminated. respectively. Combining hydraulic permeability versus time with modified fouling index (MFI) determination and membrane autopsy (FEG-SEM and AFM observations), it is observed that fouling is mainly due to cake formation (C) 2009 Elsevier B V All rights reserved