A bacterial strain, designated AST4(T), was isolated from activated sludge. The bacterium did not show significant growth on nutrient broth, but growth was clearly stimulated by addition of supernatant from other bacterial cultures. Culture filtrate of a strain related to the genus Sphingomonas in particular increased the cell yield and growth rate of strain AST4(T). Phylogenetic analysis based on the 16S rRNA gene sequences showed that strain AST4(T) is located within the 'Rhodobacter group' in the alpha-3 subclass of Proteobacteria, but is clearly distant from related genera in this group such as Paracoccus, Rhodobacter and Rhodovulum. Strain AST4(T) is a Gram-negative, non-motile, rod-shaped (0.6-0.8x1.3-2.0 micro m) and aerobic bacterium. It was not able to reduce nitrate to nitrite or N(2). No phototrophic growth was observed. Optimal growth occurred at 30 degrees C and pH 6.5-7.5. The dominant cellular fatty acid in the isolate was C(18 : 1)cis11. Ubiquinone-10 was the major respiratory quinone. The G+C content was 64.5 mol% (by HPLC). Based on the phylogenetic and phenotypic traits, the name Catellibacterium nectariphilum gen. nov., sp. nov. is proposed for this isolate; the type strain is AST4(T) (=NBRC 100046(T)=JCM 11959(T)=DSM 15620(T)).