CW-4Y was identified as Stenotrophomonas sp. by morphological and physiological characteristics, and phylogenetic analysis of its 16S rDNA gene sequence. Nitrogen removal by CW-4Y was analyzed in relation to the ammonium concentration, presence of organic carbon, carbon source, and carbon-to-nitrogen ratio (C/N). Stenotrophomonas CW-4Y has heterotrophic nitrification and aerobic denitrification abilities. Stenotrophomonas CW-4Y utilized only glucose as carbon sources, and heterotrophic nitrification and aerobic denitrification were observed regardless of the type of nitrogen source. The maximum ammonium removal rate of CW-4Y was 80 $mg-N{cdot}L^{-1}{cdot}d^{-1}$ and its denitrification rate of 192 $mg-N{cdot}L^{-1}{cdot}d^{-1}$ at $NO_3{^-}-N$ (about 280 ppm) in shake culture experiments at a C/N ratio of about 15 was about 30 times higher than those of other bacteria with the same ability.