Long-term inXuences of diVerent steady-state pH conditions on microbial community composition were determined by Xuorescence in situ hybridization (FISH) in a laboratory scale reactor conWgured for enhanced biological phosphorus removal (EBPR). Chemical proWles were consistent with shifts in populations from polyphosphateaccumulating organisms (PAO) to glycogen-accumulating organisms (GAO) when pH fell from pH 7.5 to 7.0 and then to 6.5. While biomass was both dispersed and Xocculated at pH 7.5, almost complete granulation occurred gradually after pH was dropped to 7.0, and these granules increased in size as the pH was reduced further to 6.5. Reverting back to pH 7.5 led to granule breakdown and corresponding increases in anaerobic phosphate release. Granules consisted almost entirely of Accumulibacter PAO cells, while putative GAO populations were always present in small numbers. Results suggest that low pH may contribute to granulation under these operational conditions. While chemical proWles suggested the PAO:GAO balance was changing as pH fell, FISH failed to reveal any marked corresponding increase in GAO abundances. Instead, TEM evidence suggested the Accumulibacter PAO phenotype was becoming more like that of a GAO. These data show how metabolically adaptable the Accumulibacter PAO can be under anaerobic:aerobic conditions in being able to cope with marked changes in plant conditions. They suggest that decreases in EBPR capacity may not necessarily reXect shifts in community composition, but in the existing population metabolism.