This review discusses a group of bacteria, the 'G-bacteria', which have a distinctive morphology of cocci in tetrads, sheets or clusters, that are seen in large numbers in many activated sludge biomass samples. Isolates of 'G-bacteria' that have been grown axenically are phylogenetically diverse. The Gram-negative members include several alpha- and beta-proteobacteria, among which is the genus Amaricoccus, while the Gram-positive 'G-bacteria' contain several members of the actinobacteria. It is probable that other, as yet uncharacterized, 'G-bacteria' exist in activated sludge. The hypothesis that these 'G-bacteria' are detrimental to the process of enhanced biological phosphate removal by competing for substrates anaerobically with the phosphate-accumulating bacteria in such systems, based as it is largely on mixed-culture studies, receives little support from studies using those available in pure culture. The evidence on which these conclusions are founded is discussed, as are the arguments used to explain why these 'G-bacteria' all appear to thrive under conditions found in certain activated sludge systems.