In full-scale ultrafiltration plants there is a strong need to observe fouling in an early stage. The operator uses the transmembrane pressure (TMP). In order to compare different temperatures and different fluxes, it is better to have a criterion based on resistance or normalised flux. But also other red alerts are possible. For instance the fouling rate defined as the resistance increase during one day of operation. A fast increase is of course a red alert for the operator to change the operating conditions. Another proposed red alert is the slope of the filtration curve between two backwashes. An increase in this slope can be an indication that the available filtration area is decreasing because of accumulating fouling. The first three criteria alerted the operator four days after the event. The fourth criterion (slope of the filtration curve) alerted already after one day and it is a possible new early warning system for fouling. The drawback of this parameter is that it can also be influenced by the water quality. Perhaps a combination of the fouling rate and the slope of the filtration curve will give the operator the best information possible at this moment.