The diversity and dynamics of microbial communities of phosphorus-removing and non-phosphorus-removing activated sludge systems have mostly been analysed by culture-dependent methods. A more direct method is the isolation of the total protein content of samples of activated sludge systems and separating proteins with SDS-polyacrylamide gel electrophoresis (SDS-PAGE). Total proteins were analysed and used as fingerprints to type and compare the diversity of the bacterial community. The objectives of this study were to determine if there were any differences between the anaerobic, anoxic and aerobic zones of an activated sludge plant as well as the effect of seasonal changes on the bacterial community structure of an activated sludge plant over a 34-week period. The protein profiles indicated a relatively high (>63%) similarity between the samples. The results indicated no specific protein pattern in the different zones or due to seasonal changes. This implicated that a stable bacterial community was present throughout the study period.