Proliferation of glycogen accumulating organisms (GAO) has been identified as a potential cause of enhanced biological phosphorus removal (EBPR) failure in wastewater treatment plants (WWTP). GAO compete for substrate with polyphosphate accumulating organisms (PAO) that are the microorganisms responsible for the phosphorus removal process. In the present article, the effects of temperature on the anaerobic metabolism of GAO were studied in a broad temperature range (from 10 to 40 degrees C). Additionally, maximum acetate uptake rate of PAO, between 20 and 40 degrees C, was also evaluated. It was found that GAO had clear advantages over PAO for substrate uptake at temperatures higher than 20 degrees C. Below 20 degrees C, maximum acetate uptake rates of both microorganisms were similar. However, lower maintenance requirements at temperature lower than 30 degrees C give PAO metabolic advantages in the PAO-GAO competition. Consequently, PAO could be considered to be psychrophilic microorganisms while GAO appear to be mesophilic. These findings contribute to understand the observed stability of the EBPR process in WWTP operated under cold weather conditions. They may also explain the proliferation of GAO in WWTP and thus, EBPR instability, observed in hot climate regions or when treating warm industrial effluents. It is suggested to take into account the observed temperature dependencies of PAO and GAO in order to extend the applicability of current activated sludge models to a wider temperature range.