A microcosm study was conducted to test the bioremediation potential of Paracoccus sp. strain HPD-2 on an aged PAH-contaminated soil. Bioaugmented microcosms showed a 23.2% decrease in soil total PAH. concentrations after 28 days, with a decline in average concentration from 9942 to 7638 mu g kg(-1) dry soil. The percentage degradation of 3-, 4- and 5(+6)-ring PAHs was 35.1%, 20.7% and 24.3%, respectively. Higher counts of culturable PAH-degrading bacteria, microbial biomass and enzyme activities were observed in bioaugmented soil. The bioaugmented microcosms showed significant increases (p < 0.05) in the average well-color development (AWCD) obtained by the BIOLOG ecoplate assay and Shannon-Weaver index (H) compared to the controls. Principal component analysis of BIOLOG data clearly differentiated between the bioaugmented and control microcosms, implying that bioaugmentation restored the microbiological functioning of the PAH-contaminated soil. The results suggest that bioaugmentation by Paracoccus sp. strain HPD-2 may be a promising bioremediation strategy for aged PAH-contaminated soils. (C) 2009 Elsevier Ltd. All rights reserved.