Two Gram-negative, rod-shaped, motile, aerobic bacterial strains designated HY-14(T) and HY-24 were isolated from the sludge of an anaerobic ammonium-oxidizing bioreactor. The strains could not grow with 5 % (w/v) NaCl, did not produce acid from d-glucose or utilize d-glucose, gluconate or citrate as a sole carbon source. Summed feature 3 and C(16 : 0) were the most abundant fatty acids; hydroxyl fatty acids C(12 : 0) 3-OH and C(10 : 0) 3-OH were also present. Fatty acid C(12 : 0) 2-OH was absent. The DNA G+C contents of strains HY-14(T) and HY-24 were 50.6+/-0.5 mol%. Phylogenetic analyses based on 16S rRNA gene sequences showed that strains HY-14(T) and HY-24 formed a monophyletic clade within the genus Pseudomonas. The highest sequence similarities were to Pseudomonas pseudoalcaligenes DSM 50188(T) (95.9 %). On the basis of phenotypic and phylogenetic properties, strains HY-14(T) and HY-24 are proposed as a novel species of the genus Pseudomonas, for which the name Pseudomonas caeni sp. nov. is proposed. The type strain is HY-14(T) (=KCTC 22292(T)=CCTCC AB208156(T)).