A strain D3 of denitrifying bacterium was isolated from an anammox reactor, and identified as Pseudomonas mendocina based on the morphological and physiological assay, Vitek test, Biolog test, (G + C) mol% content, and 16S rDNA phylogenetic analysis. As a typical denitrifying bacterium, strain D3 achieved the maximal nitrate reduction rate of 26.2 mg/(L x d) at the nitrate concentration of 88.5 mg N/L. The optimal pH and growth temperature were 7.84 and 34.9 degrees C, respectively. Strain D3 was able to oxidize ammonia under anaerobic condition. The maximum nitrate and ammonium utilization rates were 6.37 mg/(L x d) and 3.34 mg/(L x d), respectively, and the consumption ratio of ammonia to nitrate was 1:1.91. Electron microscopic observation revealed peculiar cell inclusions in strain D3. Because of its relation to anammox activity, strain D3 was presumed to be anammoxosome. The present investigation proved that denitrifying bacteria have the anammox ability, and the results have engorged the range of anammox populations.