Effects of the properties of flocs consisting of natural organic matter (NOM) contained in surface waters on membrane fouling in the coagulation-microfiltration (MF) treatment were examined in this study. Two types of waters containing different quantities of dissolved NOM primarily composed of humic substances were employed, low humic natural water and high humic lab-prepared water. Physical properties of the flocs formed in the coagulation process were analyzed by flocculation index (FI), fractal dimension, and microscopic observations. Flocs of high humic water were small and regular in shape, represented by the low FI and high fractal dimension, indicating that these flocs can be more easily compressible than those of low humic natural water. This was confirmed by direct microscopic observation and increased compressibility of the cakes formed with the high humic water flocs on the membrane surface. As a result, membrane fouling became pronounced, supported by increase of specific cake resistance and substantial decrease of water flux through the membrane. It is our expectation that these results provide useful information for the control of membrane fouling caused by flocs in the coagulation-MF membrane treatment. (c) 2008 Elsevier B.V. All rights reserved.