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A B S T R AC TIn this study, the characteristics of extracellular polymeric substance (EPS) and the mechanism of fouling formation investigated using a submerged membrane bioreactor system consisting of a polytetrafl uoroethylene (PTFE) membrane. During 85 days of operation, the bound EPS observed as the main component, which contributed major part of resistance to fouling. The protein to carbohydrate ratio was higher in the growth phase than in the stationary phase. The fouling resistances contributed by each individual resistances, i.e. loose cake layer, dense cake layer and internal fouling evaluated by batch fi ltration test after rinse, wash, and backwash operations respectively. The results showed that fouling of membrane and transmembrane pressure increased very quickly within 30 min of operation mainly due to resistance offered by immediate cake layer formation. The analysis of EPS contributed by different fouling layer revealed that most of the carbohydrate adsorbed quickly on the PTFE membrane fi rst and then protein concentration increased with increasing fi ltration time. The protein content of EPS gradually increased when the foulant approach the membrane, whereas carbohydrate content showed opposite trend. Furthermore, the fi ltration of pure bacteria (Ralstonia) showed that most of proteins adsorbed on the membrane surface were hydrophobic and identifi ed its contribution as the biological functions of translation.