The aim of this study was to evaluate the ultrafiltration (UF) performance for removing Microcystis aeruginosa cells under different growth ages (1, 2, 3 and 4 months old). Special attention was given to cell damaging and subsequent release of microcystins to permeate. Experiments were performed with a hollow-fibre cellulose acetate membrane (100 kDa). UF achieved an absolute removal of M. aeruginosa single cells, producing chlorophyll-a free water and with a turbidity below 0.1 NTU. Cell lysis occurred at all cell growth phases although greater damage was observed for older cultures. However, the permeate quality was never deteriorated and its microcystin concentration was always identical or lower than the dissolved concentration in the feed water. The hydrophilic UF membrane presented low adsorption of microcystins but the microcystin rejection increased in the presence of algogenic organic matter (AOM). The type rather than the overall concentration of salts and organics ruled the membrane fouling, the 1-month-old suspension (polysaccharide-rich AOM with scaling multivalent ions) presenting higher fouling potential than the 3-month-old suspension (protein-rich AOM with much lower content of multivalent ions). (C) 2009 Elsevier B.V. All rights reserved.