A group of Gram-negative denitrifying bacteria has been isolated from a denitrifying reactor treating landfill leachate. The new isolates produced both oxidase and catalase and showed growth on acetate, butyrate, n-caproate, i-butyrate, i-valerate, propionate, n-valerate, lactate, alanine, benzoate, phenylalanine and ethanol. No growth was observed on sugars. The bacteria could perform anoxic reduction of nitrate, nitrite and nitrous oxide to nitrogen, coupled to the oxidation of the same substrates as those used under aerobic conditions, except for aromatic compounds. They were very efficient denitrifiers, as estimated from the specific rate of N2 gas production. All the strains showed the same 16S rDNA restriction profile and one of them, designated 23310T, was selected for phylogenetic analysis. The organism clustered within the family Comamonadaceae, being related to Comamonas terrigena (95.8% sequence similarity). On the basis of the phylogenetic analysis, physiological characterization and the ability to efficiently reduce nitrate to N2, it is proposed that the bacterium be assigned to a new species, Comamonas nitrativorans. The type strain is 23310T (= DSM 13191T = NCCB 100007T = CCT 7062T).