A standard flux-step method has been developed for assessing fouling in a membrane bioreactor operating at constant flux. Three keys parameters based on transmembrane pressure (TMP) were derived to depict fouling behaviour and identify the onset of fouling at the so-called critical flux. A zero rate of TMP increase (dP/dt) was never attained during the trial, such that no critical flux, in its strictest definition, could be defined in this study for a submerged MBR challenged with real and synthetic sewage. The three TMP-based parameters all indicated roughly the same flux value at which fouling started to become significant (the weak form of critical flux), this being 10 and 181 m(-2) h(-1) when the MBR was fed with synthetic and real sewage, respectively. Long-term filtration trials featured much lower dP/dt values compared to those obtained during short-term fouling test, but sub-critical flux operation led to a catastrophic increase in fouling rate after some critical time period apparently dependent on feedwater matrix and flux. It was concluded that flux-step determination of the critical flux cannot be used to predict long-term TMP behaviour in real MBR systems, but nonetheless provides useful data on comparative fouling propensity. (C) 2003 Elsevier B.V. All rights reserved.