Membrane filtration has become an accepted process for potable water production, but membrane fouling remains a significant problem. Natural organic matter (NOM) and in particular residues of microbial activities are frequently cited as one of the major membrane foulant. The objective of this work was to evaluate the fouling properties of bacterial cell wall residues isolated from surface waters and Extracellular Polymeric Substances (EPS) isolated from algae. In addition alginic acid (polymer and dimer) were also studied. Some humic materials (NOM fractions isolated from rivers and commercial humic substances) were also studied for comparison. The experimental approach was based on both flat sheet cell membrane tests (bacterial cell wal residues, alginic acid) and hollow fiber membrane tests (EPS). Two 100 KDa flat sheet membranes (regenerated cellulose membrane - YM and polyethersulfone membrane - PES) and 100 KDa polysulfone hollow fibers (ID : 380 µm ; ED : 720 µm, 0.02 m²) were selected for this study. Flux decline under constant TMP was monitored as an indication of fouling. The first part of this study was dedicated to the isolation and characterization of the isolated microbial residues. Bacterial cell wall residues (i.e. fraction of NOM retained in a 3.5 KDa dialysis bag after 1 µm filtration) were isolated from two natural waters according to the protocol proposed by Leenheer et al (2000). A battery of analytical tools (FTIR, NMR, Fluorescence, HPSEC, Pyrolysis-GC/MS, total amino acid and amino sugars) confirmed that this fraction of NOM (20 to 30 % of the TOC) is comprised of a mixture of polysaccharides (aminosugars) and proteins as predominant biopolymers. In the second part of the study, a large diversity of analytical tools were developed in order to characterize the morphology (AFM, FESEM) and physical-chemical properties (Contact angle, streaming potential) of the selected membranes. The two flat sheet membranes showed significant differences in terms of hydrophobic/hydrophilic character, zeta potential (negatively charged membranes), roughness and pore density. The hollow fibers were found to exhibit a negatively charged surface. In the third phase of this study, the foulant properties of the selected microbial residues were evaluated. Experiments with natural waters were also conducted. Organics were resolubilized (bacterial cell wall residues and alginic acid) or diluted (EPS) in MilliQ water (DOC : 5 to 10 mg/L C) and filtered through the membranes (TMP : 1 bar for the flat sheet membrane, TMP : 0.45 bar out/in filtration for the hollow fibers) at neutral pH. Whatever the membrane used, solutions of bacterial cell wall residues, EPS and polymeric alginic acid showed very severe fouling properties as compared to humic materials. Analyses conducted on permeates indicated a significant reduction of the DOC content with a predominant retention of higher molecular weight structures (HPSEC analyses). Includes figures.