Four Gram-negative, rod-shaped, non-spore-forming, denitrifying isolates were obtained from the activated sludge of an aerobic-anaerobic wastewater treatment plant in Belgium. Analysis of repetitive sequence-based PCR showed that the four isolates were genetically different from each other. Results of 16S rRNA gene sequence analysis and DNA-DNA hybridization experiments indicated that these four isolates were affiliated to the genus Acidovorax and could be differentiated from all recognized species of the genus. Analysis of whole-cell proteins and results of physiological and biochemical tests allowed differentiation of the new isolates from their closest phylogenetic neighbours. These new isolates therefore represent a novel species of the genus Acidovorax, for which the name Acidovorax caeni sp. nov. is proposed. The type strain is R-24608(T) (=LMG 24103(T) =DSM 19327(T)).