A survey of several enhanced biological phosphorous removal (EBPR) plants within Australia has demonstrated that a group of bacteria known as the "G" bacteria are able to proliferate and out-compete phosphorous accumulating organisms (PAOs) under a broad range of plant configurations. The diverse designs and operational parameters of these plants did not permit definitive determination of the factor(s) contributing to the proliferation of G bacteria. However there were a number of common operational aspects which had lead to some explanations for "G" bacteria being able to outcompete the PAOs. Two plants were monitored over time to assess the G bacteria and PAO populations and compare with operational parameter. The mixed liquor biomass, operational parameters (particularly dissolved oxygen concentration and substrate available in the anaerobic zone ) were compared to other plants successfully and unsuccessfully reducing phosphorous from the wastewater. Two critical factors recognised in this study were the dissolved oxygen concentration in the aerobic zone and the type and amount of carbon source in the anaerobic zone.