A gram-negative coccus, designated strain Ben 117T, was obtained in axenic culture by micromanipulation from an Australian activated sludge biomass sample, which had been subjected to chlorination in order to alleviate problems associated with foaming and bulking. This isolate was a strict aerobe and grew in axenic culture, also appearing in biomass samples as cocci or clusters of cocci in tetrads, thus resembling the morphotype 'G-bacteria' seen commonly in activated sludge samples. Strain Ben 117T was non-motile, aerobic, oxidase-negative and catalase-positive and grew between 15 and 30 degrees C, with an optimum of 25-30 degrees C. The pH range for growth was between 6.0 and 8.5, with an optimum of 7.5-8.5. The isolate stained positively for intracellular polyphosphate and poly-beta-hydroxybutyrate and its G+C content was 67 mol%. 16S rDNA sequence analysis suggests that strain Ben 117T is phylogenetically different from members of the genera Amaricoccus, gram-negative 'G-bacteria' isolated previously in this laboratory. Ben 117T is a member of the Rhodocyclus group in the beta-Proteobacteria and equidistantly placed (similarity value of 95%) between Ferribacterium limneticum and Dechloromonas agitata (mean similarity value of 92% with the genus Rhodocyclus). Based on phenotypic and phylogenetic evidence, it is proposed that strain Ben 117T be designated a novel species in a new genus, Quadricoccus australiensis gen. nov., sp. nov.; the type strain is Ben 117T (= NCIMB 13738T = CIP 107055T).