As a p-xylene (p-Xyl)-degrading microorganism, Pseudomonas sp. NBM21 was isolated from an activated sludge of a wastewater treatment plant. NBM21 degraded p-Xyl, m-xylene, benzene and toluene, but not o-xylene, ethylbenzene (Eb) and styrene. NBM21 was inoculated to a biofilter with Biosol as a packing material and p-Xyl removal was operated for 105 d under sterile and nonsterile conditions. The maximum elimination capacities for p-Xyl at higher than 90% removal efficiency were 160 g/m(3)/h and 150 g/m(3)/h under nonsterile and sterile conditions, respectively. A high load of Eb adversely affected to the removal of xylene.