The biodegradation of polycyclic aromatic hydrocarbons (PAHs) (8.15 mg PAHs kg(-1) soil) in aged contaminated soil by isolated microbial consortium (five fungi and three bacteria) during the incubation of 64 d is reported. The applied treatments were: (1) biodegradation by adding microbial consortium in sterile soils (BM): (2) biodegradation by adding microbial consortium in non-sterile soils (BMN): and (3) biodegradation by in situ "natural" microbes in non-sterile soils (BNN). The fungi in BM and BMN soils grew rapidly 0-4 d during the incubation and then reached a relative equilibrium. In contrast the fungi in BNN soil remained at a constant level for the entire time. Comparison with the fungi, the bacteria in BNN soils grew rapidly during the incubation 0-2 d and then reached a relative equilibrium, and those in BM and BMN soils grew slowly during the incubation of 64 d. After 64 d of incubation, the PAH biodegradations were 35%, 40.7% and 41.3% in BNN. BMN and BM, respectively. The significant release of sequestrated PAHs in aged contaminated soil was observed in this experiment, especially in the BM soil. Therefore, although bioaugmentation of introduced microbial consortium increased significantly the biodegradation of PAHs in aged contaminated soil with low PAH concentration, the creation of optimum of the environmental situation might be the best way to use bioremediation successfully in the field. Crown Copyright (C) 2009 Published by Elsevier B.V. All rights reserved.