Sludge reduction in a conventional activated sludge (CAS) process combined with a recycled sludge reactor where Tubifex tubifex (one of Oligochaeta) was inoculated was investigated in this study. The results showed sludge production could be reduced through T. tubifex's predation on sludge in the recycled sludge reactor. The sludge reduction rate of T. tubifex (R) was from 0.18 to 0.81 mg-VSS mg-Tubifex(-1) d(-1). The sludge reduction capacity of the recycled sludge reactor E was from 650 to 1080 mg-VSS L(-1) d(-1). The optimum density of T. tubifex was 2500 mg L(-1) and the optimum sludge recycled ratio was 1. The existence of T. tubifex did not affect COD and NH(4)(+)-N removals in the process, but led to a slight decrease in TP removal. SVI almost did not change when the T. tubifex density was lower than 3300 mg L(-1). The LC50 values on T. tubifex of copper and ammonia were 2.5 and 880 mg L(-1), respectively, both of which were higher than those on Aeolosoma hemprichi.