The primary objective of this study was to examine the toxic effects of PCP on activated sludge and to analyze its metabolic responses while treating wastewater containing pentachlorophenol (PCP) in a sequencing batch reactor (SBR) system operating under different control strategies. This study was conducted in two phases 1 and 2 (8-㏊ and 12-㏊ cycles) Each phase was operated with two control strategies I and II. Strategy I (reactor 1) involved rapid addition (5 minutes to complete) of substrate to the reactor with continuods mixing but no aeration for 2 hours. Strategy II (reactor 2) involved adding the feed continuodsly during the first 2 hours of the cycle when the system, was mixed but not aerated During both phases each reactor was operated at a sludge age of 15 days. The synthetic wastewater was used as a feed. The COD of the feed solution was about 380㎎/ℓ. After the reference response for both reactors was established, the steady state response of each system was established for PUP teed concentrations of 0.1 ㎎/ℓ, 1,0㎎/ℓ, and 5.0 ㎎/ℓin SBR systems operating on both 8-hr and 12-hr cycles. Soluble COD removal was not inhibited at any feed PCP concentrations used. At 5.0 ㎎/ℓfeed PCP concentration and in SBR systems operating on phase 2, the concentrations of MLVSS were dereased; selective pressure on the mixed biomass might be increased, narrowing the range of possible ecological responses; the settleability of activated sludge was poor; the SOURs were increased, showing that the systems were shocked. Nitrification was made to some extent at all concentrations of feed PCP in SBR systems operating on phase 2 whereas in SBR systems operating on phase I little nitrification was observed, Then, nitificaiion will be delayed as much as soluble COD removal is retarded due to PCP inbition effects. Enhanced biological phosphorus removal occurring in the sytem operating with control strategy I during phase I of this work and in the presence of low concentrations of PCP was unreliable and might cease at anytime, whereas enhanced biological phosphorus removal occurring in the system operating with either control strategy . I or II during phase 2 of this work and in the presence of feed PCP concentrations up to 1.0 ㎎/ℓ was reliable. When, however, such processes were exposed to 5.0 ㎎/ℓ PCP dose, enhanced phosphorus removal ceased and never returned.