Three strains of Gram-negative bacteria designated strains H2(T), H6, and H7 were isolated from bioreactors that degraded the herbicide hexazinone. Similar morphological characteristics, cellular fatty acid profiles, and 16S rRNA gene sequences show that the isolates are members of the same species. These characteristics also show that the isolates belong to the genus Pseudomonas with P. graminis, P. putida, and P. stutzeri as close relatives. The 16S rRNA gene of the H2(T) strain differed from that of type strains for P. graminis, P. putida, and P. stutzeri by 1.9, 2.5, and 2.7 %, respectively, indicating that the H2(T), H6, and H7 strains are related to P. graminis, P. putida, and P. stutzeri but are different enough to represent a novel species. The G+C content of the three strains averaged 61.2 +/- A 0.8 mol% which is similar to the values reported for P. graminis (61), P. putida (61.6), and P. stutzeri (62.2-65.5). The major cellular fatty acids present in the H2(T) strain were C-18:1 omega 7c/C (18:1) omega 6c (34.3 %), C-16:1 omega 6c/C-16:1 omega 7c (27.4 %), C-16:0 (20.6 %), C-12:0 (7.9 %), C-12:0 3-OH (4.5 %), and C-10:0 3-OH (3.1 %). The name Pseudomonas kuykendallii sp. nov. is proposed for these bacteria.