A group of yellow-pigmented isolates from ammonia-supplied biofilters showed an unusual denitrification reaction. All strains reduced nitrite but not nitrate without production of nitrogen (N-2). The only product found was nitrous oxide (N2O). The strains were divided into two clusters and one separate strain by their fatty acid profiles, which were similar to the fatty acid profiles of the genera Xanthomonas and Stenotrophomonas. Analyses of the 16S rDNA sequences showed that these clusters and the separate strain form three independent lines within the Xanthomonas branch of the The evolutionary distances of the isolates to members of the related genera Xanthomonas, Stenotrophomonas and Xylella calculated by the 16S rDNA sequences led to the proposal of two new genera and three new species, Stenotrophomonas nitritireducens sp. nov., Luteimonas mephitis gen. nov., sp. nov. and Pseudoxanthomonas broegbernensis gen. nov., sp. nov. The type strains are Stenotrophomonas nitritireducens L2(T) (= DSM 125753(T)), Luteimonas mephitis B1953/27.1(T) (= DSM 12574(T)) and Pseudoxanthomonas broegbernensis B1616/1(T) (= DSM 12573(T)).