Polyhydroxyalkanoates (PHAs) are promising biodegradable polymers The use of mixed microbial cultures (MMC) and low cost feedstocks have a positive impact on the cost-effectiveness of the process It has typically been carried out in Sequencing Batch Reactors (SBR) In this study, a 2-stage CSTR system (under Feast and Famine conditions) was used to effectively select for PHA-storing organisms using fermented molasses as feedstock The effect of influent substrate concentration (60-120 Cmmol VFA/L) and HRT ratio between the reactors (0.2-0.5 h/h) on the system's selection efficiency was assessed. It was shown that Feast reactor residual substrate concentration impacted on the selective pressure for PHA storage (due to substrate-dependent kinetic limitation) Moreover, a residual substrate concentration coming from the Feast to the Famine reactor did not Jeopardize the physiological adaptation required for enhanced PHA storage The culture reached a maximum PHA content of 61% This success opens new perspectives to the use of wastewater treatment infrastructure for PHA production, thus valorizing either excess sludge or wastewaters (C) 2010 Elsevier Ltd. All rights reserved.