Microbial consortia capable of degrading 3-chlorophenol (3-CP) were enriched in continuous up-flow column reactors under circum-denitrifying conditions. 3-CP degradation capability was developed and sustained when 3-CP was supplied at 16-21 muM, although suppression of the 3-CP degradation capability was observed when 3-CP was supplied at 42 muM. When 3-CP was stably degraded, the ratio of nitrate consumption to 3-CP degradation approached the theoretical stoichiometric value, which was calculated by assuming a 3-CP de gradation-dependent nitrate reduction. Batch-incubation experiments demonstrated that the microbial consortium that was enriched in the column reactors required either nitrate or oxygen for degrading 3-CP, while 3-CP was not degraded under sulfate-degrading conditions. Although many attempts were made to sustain the microbial 3-CP degradation capability under denitrifying conditions, mostly in batch cultures, none of them have been successful so far. Therefore, the results obtained in this study may be the first to demonstrate sustainable 3-CP degradation capability under circumdenitrifying conditions.