Routine microscopic examination of biomass samples from many activated sludge plants around the world reveals the presence of large numbers of both Gram positive and Gram negative coccoid cells often arranged in distinctive tetrads or sheets of tetrads. Three Gram negative tetrad cocci were successfully isolated into pure culture using micromanipulation from samples of biomass from plants in Tamworth, N.S.W., Australia, Verona, Italy and Macau. After extensive phenotypic characterisation and 165 rDNA sequence analysis, all isolates showed high overall similarity to each other, but were phylogenetically quite different to any previously sequenced bacterium. They have therefore been classified as three species of a new genus, the genus Amaricoccus in the α Proteobacteria in the domain Bacteria. None could accumulate polyphosphate granules, and production of intracellular poly-β-hydroxybutyrate (PHB) granules depended on the carbon source used in the medium. A similar characterisation of the original isolate of the G-bacteria described by Cech and Hartman showed this organism also belonged to the same genus as the other three isolates, but as a distinct species. The practical implications of these observations are discussed.