Three isolates of gram-negative bacteria, strains Ben 102T, Ben 103T, and Ben 104T, were obtained in pure culture by micromanipulation from activated sludge biomass from wastewater treatment plants in Italy, Australia, and Macau, respectively. These isolates all had a distinctive morphology; the cells were cocci that usually were arranged in tetrads. Based on this criterion, they resembled other bacteria from activated sludge previously called "G" bacteria. On the basis of phenotypic characteristics and the results of 16S ribosomal DNA sequence analyses, the three isolates were very similar to each other, but were sufficiently different from their closest phylogenetic relatives (namely, the genera Rhodobacter, Rhodovulum, and Paracoccus in the alpha subdivision of the Proteobacteria) to be placed in a new genus, Amaricoccus gen. nov. Each of the three isolates represents a new species of the genus Amaricoccus; strains Ben 102T, Ben 103T, and Ben 104T are named Amaricoccus veronensis, Amaricoccus tamworthensis, and Amaricoccus macauensis, respectively. An isolate designated Ben 101T, which was isolated independently by Cech and Hartman in Kaplice, Czech Republic, was also characterized and belongs to the same genus. We propose that the isolate of Cech and Hartman should be placed in another new species, Amaricoccus kaplicensis.