An important step in the battle against filamentous bulking is the development of a monitoring system for activated sludge properties. Therefore, a fully automatic image analysis method for recognizing and characterizing flocs and filaments in activated sludge images has been developed. This procedure has been subsequently used to monitor activated sludge properties in a lab-scale installation. The results of a 100-days experiment indicate that the image information correlates well with the evolution of standard settling properties, in this case the Sludge Volume Index. It is shown that, at the onset of severe filamentous bulking, there is an increase in total filament length on the one hand, and a significant change in floc shape on the other hand.