Candidatus Accumulibacter phosphatis (CAP) is an unclassified Betaproteobacteria that is a common bacterial community member of wastewater treatment plants performing enhanced biological phosphorus removal (EBPR) and is a polyphosphate-accumulating organisms. The role of CAP in EBPR was elucidated using culture-independent approaches such as 16S rRNA clone banks that showed the Betaproteobacteria dominated lab-scale EBPR reactors. Further work using clone banks and fluorescence in situ hybridization identified a group of bacteria, closely related to Rhodocyclus as the dominant member of lab-scale communities.