his study investigates the relationship between biodiversity and functional resistance of nine activated sludge wastewater treatment plants to three selected toxicants: cupric sulfate (Cu[II]), 3,5-dichlorophenol (3,5-DCP), or 4-nitrophenol (4-NP). Resistance was measured as the 30 min IC50 or the concentration of toxicant that resulted in 50% reduction in oxygen utilization rate (OUR). Biodiversity was assessed using polymerase chain reaction-terminal restriction fragment length polymorphism (PCR-T-RFLP) and PCR-denaturing gradient gel electrophoresis (PCRDGGE) both targeting the 16S ribosomal RNA gene. Statistical analysis of 30 min IC50 and TRFLP data showed a significant positive correlation.