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To study the effect of zinc on the biodegradation of phenanthrene by the microbial biomass in soil.Uncontaminated soil was amended with zinc and phenanthrene as single or co-contaminants, and microbial metabolic activity was measured using an intracellular dehydrogenase enzyme bioassay over 37 days. Contaminants were amended at optimum, action and double the action level specified in ‘The New Dutch List’ (Ministry of Housing, Spatial Planning and Environment, the Netherlands, 2000). Microbial activity in soils with zinc or phenanthrene alone indicated the presence of tolerant, albeit inhibited soil micro-organisms. A zinc concentration at the optimum level of 140 mg kg−1 in the co-contaminated soil (phenanthrene at 40 mg kg−1) resulted in marginal stimulation of the rate of phenanthrene biodegradation. However, Zn2+ concentrations at the action and double the action level of zinc (720 and 1440 mg kg−1) inhibited phenanthrene degradation.Biodegradation of phenanthrene in soils co-contaminated with zinc at concentrations above the action value is impeded.Bioremediation efforts to remove polycyclic aromatic hydrocarbon in zinc co-contaminated soils are likely to be constrained.