p-Nitrophenol (PNP) -degrading bacteria were isolated from a reactor using a mineral salt medium containing a low and high PNP concentration. We isolated two bacterial species, Pseudomonas sp. YTK17 and Rhodococcus opacus YTK32, that utilize PNP as their sole source of carbon and energy. These strains exhibited differences in PNP degradation activity in relation to PNP concentration. Strain YTK17 showed a high level of degradation following pre-exposure to a low PNP concentration, whereas strain YTK32 required a relatively high PNP concentration for degradation to occur. These results indicated that phylogenetically and physiologically different types of PNP-degrading bacteria coexisted in a reactor.