Effects of heavy metals on phenol-utilizing microorganisms that could co-metabolically degrade naphthol were investigated. Polymerase chain reaction (PCR) and denaturing gradient gel electrophoresis (DGGE) identified these phenol-utilizing microorganisms as Alicycliphilus denitrificans K601, Alicycliphilus sp. R-2461, uncultured Alicycliphilus sp., and Acidovorax aerodenitrificans. Inhibitory effects of Pb, Cu, Cd, and Zn on phenol biodegradation were in the order of Cu > Cd > Pb > Zn. Inhibitory effects of mixed heavy metals on phenolutilizing microorganisms were in the order of (Cd + Pb) ≈ (Cd + Cu) > (Zn + Pb) > (Zn + Cd) > (Pb + Cu) > (Zn + Cu). The presence of mixed heavy metals synergistically inhibited microbial degradation of phenol. The relationship between reaction constants and inhibition concentrations that caused 50% of the degradation rate (IC 50 ) for mixed heavy metals was derived as k 2 = 0.69 • IC 50 -1.007 . In this study, heavy metals inhibited phenol biodegradation through synergistic interactions. However, other studies have reported antagonistic interactions depending on the microbial community and heavy metals, indicating that the inhibition type of heavy metal on microbial degradation depends on the microorganism and the type of heavy metal.