A Gram-positive bacterium, designated strain AG019(T), was isolated by micromanipulation from aerobic granules obtained from a laboratory-scale sequencing batch reactor. This isolate grew axenically as cocci clustered predominantly in tetrads, and was morphologically similar to the dominant organisms observed in the biomass. The morphology also resembled that of the tetrad-forming organisms commonly seen in activated sludge samples. Strain AG019(T) was found to be an oxidase-negative, catalase-positive, non-motile aerobe that does not reduce nitrate and grows at temperatures between 15 and 40 degrees C, with an optimum at 37 degrees C. The pH range for growth was 5.0-9.0, with an optimum at pH 7.5. Strain AG019(T) contained a peptidoglycan with directly cross-linked meso-diaminopimelic acid (type A1gamma) and lacked mycolic acids. The G+C content of the DNA was 75 mol%. Menaquinone MK-8(H(2)) was the major isoprenoid quinone. The bacterium stained positively for intracellular polyphosphate granules but not for poly-beta-hydroxyalkanoates. It produced capsular material and showed autoaggregation ability. Phenotypic and 16S rRNA gene analyses showed that the bacterium differed sufficiently from its closest phylogenetic relatives, namely members of the suborder Frankineae, which includes the genera Geodermatophilus, Blastococcus, Frankia, Sporichthya, Acidothermus and Microsphaera, that it is proposed that it be placed in a novel genus, Quadrisphaera, as Quadrisphaera granulorum gen. nov., sp. nov. The type strain is AG019(T) (=ATCC BAA-1104(T)=DSM 44889(T)).