Enhanced biological phosphorus removal (EBPR) performance is directly affected by the competition between polyphosphate accumulating organisms (PAOs) and glycogen accumulating organisms (GAOs). This study investigates the effects of carbon source on PAO and GAO metabolism. Enriched PAO and GAO cultures were tested with the two most commonly found volatile fatty acids (VFAs) in wastewater systems, acetate and propionate. Four sequencing batch reactors (SBRs) were operated under similar conditions and influent compositions with either acetate or propionate as the sole carbon source. The stimulus for selection of the PAO and GAO phenotypes was provided only through variation of the phosphorus concentration in the feed. The abundance of PAOs and GAOs was quantified using fluorescence in situ hybridisation (FISH). In the acetate fed PAO and GAO reactors, “Candidatus Accumulibacter phosphatis” (a known PAO) and “Candidatus Competibacter phosphatis” (a known GAO) were present in abundance. A novel GAO, likely belonging to the group of Alphaproteobacteria, was found to dominate the propionate fed GAO reactor. The results clearly show that there are some very distinctive differences between PAOs and GAOs in their ability to take up acetate and propionate. PAOs enriched with acetate as the sole carbon source were immediately able to take up propionate, likely at a similar rate as acetate. However, an enrichment of GAOs with acetate as the sole carbon source took up propionate at a much slower rate (only about 5% of the rate of acetate uptake on a COD basis) during a short-term switch in carbon source. A GAO enrichment with propionate as the sole carbon source took up acetate at a rate that was less than half of the propionate uptake rate on a COD basis. These results, along with literature reports showing that PAOs fed with propionate (also dominated by Accumulibacter) can immediately switch to acetate, suggesting that PAOs are more adaptable to changes in carbon source as compared to GAOs. This study suggests that the PAO and GAO competition could be influenced in favour of PAOs through the provision of propionate in the feed or even by regularly switching the dominant VFA species in the wastewater. Further study is necessary in order to provide greater support for these hypotheses. © 2005 Wiley Periodicals, Inc.