Membrane fouling is the most serious problem affecting membrane bioreactors (MBR). It decreases membrane permeability and consequently increases energy consumption. Various methods and techniques to measure fouling rates (FR) can be found in the literature, but few provide online data, and most are expensive or too invasive. The aim of this short communication is to present a new online method for monitoring and estimating the fouling in an MBR, based on the use of the derivative of the transmembrane pressure (TMP) per cycle, for which hourly and daily mean values can be obtained. This online method (FR per cycle) was experimentally evaluated at a lab-scale MBR plant, where fouling conditions were induced by the addition of a protein (albumin) and a polysaccharide (glucose). Finally, we compared the new method to standard methods, such as the flux-step method. Similar results for fouling rates were found, but the new method showed increased sensitivity. At the same time, the method seems to be useful in characterizing the nature of fouling. (C) 2011 Elsevier B.V. All rights reserved.