SwitzerlandInstitute for Aquatic Science and Technology, Überlandstr.The formation of stable scum (activated sludge foaming) is a serious operational problem 133, Postfach 611, CH-8600 in wastewater treatment plants (WWTP) all over the world. Although scumming seems to Dübendorf, Switzerland be related to high numbers of "Microthrix parvicella" and nocardioform actinomycetes, it is still unknown whether these organisms are necessary and sufficient for the formation of stable scum. To tackle this question, the abundance of these organisms and the scum coverage of the activated sludge basins were measured on a WWTP in Switzerland for almost two years. Using fluorescence in situ hybridization we were able to recognize "M. parvicella" as well as filamentous and non-filamentous types of nocardioform actinomycetes. A newly developed rapid quantification method allowed us to measure seasonal variations of the population. The highly variable scumming followed only partly the abundance of the filamentous types of nocardioform actinomycetes and was not correlated to "M. parvicella". Neither these organisms nor any plant operating condition could fully explain the dynamics of the scumming. The data lead us to suggest that high numbers of "M. parvicella" or nocardioform actinomycetes are not sufficient to cause scumming. The measured dynamics of the population and the scum coverage also imply that, to investigate scumming at a particular WWTP, time-series of measurements including different seasons and periods of different scumming intensities are necessary. Dynamik der Population und der Schaumbildung in einer Schweizer Kläranlage