Bench scale activated sludge reactors were acclimated to dextrose and then to low levels of pentachlorophenol. The metabolic activity of activated sludge was evaluated by its specific rate of dextrose uptake, Δs/Δt/X (measured by COD removal). Depression of the specific uptake rate, resulting from batch experiments in which the activated sludges were spiked with priority pollutants, indicated the relative inhibition caused by that toxin dosage. This study intended to determine if: (1) acclimation to low levels of PCP would provide any protection to the biomass against detrimental effects of higher shock loads of pentachlorophenol; (2) PCP-acclimation would decrease the inhibitory effect of related priority pollutants; (3) PCP would be consistently and efficiently degraded in the reactors. The practicality of this study was twofold. First, to determine the feasibility of introducing small concentrations of a toxin to the biological system of a treatment facility in order to gain protection against shock loads of that and related toxic chemicals. Secondly, to develop a rapid and easy method for evaluating the effects of a chemical load on activated sludge. The procedure was found to be applicable and it showed that acclimation of activated sludge to PCP provided protection against shock loads of pentachlorophenol as well as phenol, 4-nitrophenol, 2-chlorophenol, and 2,4,6-trichlorophenol. Gas chromatography analysis showed very little disappearance of PCP in the 1 mg PCP/L reactor; however, in the 15 mg PCP/L reactor, the penta concentration decreased to virtually zero for about a week and then it began to gradually increase. / Master of Science