Biological phosphorus removal in activated sludge systems is thought to be a result of the action of polyphosphate accumulating organisms (PAO). However, not all phosphorus removed can be accounted for by PAO. A method for the qualitative in situ characterization of PAO cell clusters and closely associated extracellular exopolymers (EPS) is described. X-ray microanalysis was performed on samples of two activated sludge plants situated in Pretoria, South Africa. Analysis was done by means of scanning electron microscopy combined with energy dispersive spectrometry (EDS). Cell clusters with associated EPS, on average, contained between 57% and 59% phosphorus, while EPS alone contained, on average, between 27% and 30% phosphorus. Results suggest that phosphorus removal in activated sludge might be due not only to PAO, but also by EPS acting as a phosphorus reservoir.