A polyphasic approach was used to characterize an aerobic, Gram-negative, rod-shaped bacterium (designated strain CC-KL-3T) isolated from a hot spring. Phylogenetic analyses based on 16S rRNA genes indicated that strain CC-KL-3T showed highest sequence similarity to Hydrogenophaga bisanensis (97.7 %) and Hydrogenophaga atypica (97.6 %) and lower sequence similarity to other species (less than 97.6 %). The levels of DNA-DNA relatedness between strain CC-KL-3T, H. bisanensis and H. atypica were estimated to be 13.0 and 8.7 % (the reciprocal value was 14.7 and 6.3 %). Strain CC-KL-3T was non-motile, without apparent flagella and able to grow between 15-42 �XC (optimal 30 �X?), pH 6.0-8.0 (optimal 7.0) and 0-2 % (w/v) NaCl (optimal 0 %). The DNA G+C content was 61.4 mol% and the major quinone system was ubiquinone (Q-8). The polyamine profile revealed the predominance of 2-hydroxyputrescine and putrescine and the dominant cellular fatty acids were C16 : 0 (28.9 %), C16 : 1�s7c/C16 : 1�s6c (41.4 %) and C18 : 1�s7c/C18 : 1�s6c (11.9 %). These data corroborated the affiliation of strain CC-KL-3T to the genus Hydrogenophaga. Based on the distinct phylogenetic, phenotypic and chemotaxonomic traits, and the results of comparative 16S rRNA gene sequence analysis, strain CC-KL-3T is considered to represent a novel species of the genus Hydrogenophaga, affiliated to the family Comamonadaceae, for which the name Hydrogenophaga aquatica sp. nov. is proposed. The type strain is CC-KL-3T (=BCRC 80937T=JCM 31216T).