Membrane fouling in membrane bioreactors (MBR) is caused by cake formation on the surface, mainly attributed to suspended solids, and also by surface adsorption connected with pore blocking, attributed to soluble components of activated sludge. In this study, long-term experiments were carried out in sub-critical conditions with frequent backwashing, and the nature of fouling was investigated. The irreversible blocking by adsorption has been identified as the major cause of fouling, as it had gradually decreased membrane permeability from 417 to 55 L m(-2) h(-1) bar(-1) in 123 days of continuous experiment without chemical cleaning. Cake resistance was small due to a low flux and high aeration intensity and it remained constant for most of the experiment. A sudden acceleration of fouling has been observed after 119 days of operation and attributed to irreversible fouling that caused local fluxes to exceed critical flux and lead the filtration into critical conditions, where suspended solids start to deposit. Faster fouling, in comparison to the filtration of normal sludge, has been observed when starving biomass had been filtered. That was attributed to floc breakage.