A process is provided for increasing the production of PHA in a mixed culture biomass. In a first stage of the process, organic material associated with a substrate is converted to volatile fatty acids. In the case of a wastewater treatment process, if the wastewater includes sufficient volatile fatty acids (VFAs) to support the process, then it is unnecessary to convert organic material to VFAs. In a second stage of the process, an anaerobic-aerobic selection process is utilized to select glycogen accumulating organisms that cause these organisms to proliferate and dominate the open mixed culture biomass. By providing relatively high organic loading in the form of VFAs in the anaerobic treatment phase of the selection process, glycogen accumulating organisms having a relatively high level of stored glycogen are produced. In a third stage, the PHA accumulation process is practiced where the glycogen rich organisms are fed VFAs under anaerobic or aerobic conditions or combinations thereof. Through the consumption of externally supplied VFAs and internally stored glycogen, relatively high levels of PHA in the biomass are produced. Thereafter PHA is separated from the residual biomass.