This research evaluated the treatability of a pentachlorophenol wastewater (PCPWW) from a wood‐preserving plant. The treatment efficiencies of a naturally occurring microbial system, a commercially available “mutant”; or specialized population, and a combination of the two, while undergoing acclimation to full‐strength PCPWW in 21 days were studied. TOC and COD removals averaged less than 35 percent. The decay rates (k‐rates) for the three biological test systems were significantly lower than those typically associated with domestic wastewater. No significant differences were observed in treatment efficiencies measured as TOC and COD.