Pentachlorophenol (PCP) is a widespread contaminate of soils and ground water throughout North America. Earlier studies have indicated that microbial biodegradation leads to the formation of intermediate metabolites which are more toxic than the parent compound. Microbial degradation is by three general pathways: dechlorination, methylation, and oxidation. The relative toxicity of PCP and 25 of its identified intermediates of microbial transformation was evaluated in the static Tetrahymena pyriformis population growth assay. Dechlorination of chlorophenols resulted in a decrease in toxicity because of a decrease in both hydrophobicity and reactivity. Moreover, dechlorination of chloroanisoles resulted in a decrease in toxicity due to a decrease in hydrophobicity. Since there was a decrease in reactivity, methylation of chlorophenols resulted in a decrease in toxicity. Oxidation of chlorophenols resulted in enhanced toxicity owing to increased reactivity and concomitant decreased hydrophobicity.