Fouling of UF membranes by humic acid particles was studied. Six types of UF membranes namely PAN 20 kDa (Rhone-Poulenc), PAN 100 kDa (Osmonics), PES 150 kDa (Microdyn-Nadir) and PS 10, 50 and 100 kDa (Permionics) were tested. Analysis of the results showed that the resistance due to the adsorption of small particles on the inner pore surfaces is much greater than that occurs due to cake layer build-up. The experimental results proved that the cake resistance is linearly proportional to the feed concentration. Comparing the results obtained for the tested membranes indicated that the specific cake resistance is inversely proportional the average pore diameter of the membrane. Studying the effect of the applied pressure showed that a compressibility coefficient is 0.63 +/- 0.1. The salt concentration in the range examined has no influence on membrane fouling. The MFI-UF fouling index was calculated and no stable value was found. A linear relationship between the MFI-UF and feed concentration was achieved. The applied pressure within the range examined has no influence on the MFI-UF index. A new criterion, I,, to overcome problems associated with the existing fouling indices based on measuring the volume of permeate collected upon filtering the suspension after a given period of time to that collected upon filtering distilled water for the same period of time and through a similar membrane was proposed. The proposed criterion is stable with time, it is linear with feed concentration, it does not involve tedious mathematical calculations and its values range between 0 and 1.