Bioremediation is a cost-effective and eco-friendly approach to decontaminate soils polluted by petroleum hydrocarbons. However, this technique usually requires a long time due to the slow degradation rate by bacteria. By applying U-tube microbial fuel cells (MFCs) designed here, the degradation rate of petroleum hydrocarbons close to the anode (<1 cm) was enhanced by 120% from 6.9 ± 2.5% to 15.2 ± 0.6% with simultaneous 125 ± 7 C of charge output (0.85 ± 0.05 mW/m2, 1 kΩ) in the tested period (25 days). Hydrocarbon fingerprint analysis showed that the degradation rate of both alkanes and polycyclic aromatic hydrocarbons (PAHs) was accelerated. The decrease of initial water content from 33% to 28% and 23% resulted in a decrease on charge output and hydrocarbon degradation rate, which could be attributed to the increase of internal resistance. A salt accumulation was observed in each reactor due to the evaporation of water from the air-cathode, possibly inhibited the activity of exoelectrogenic bacteria (EB) and resulted in the elimination of the current at the end of the tested period. The number of hydrocarbon degradation bacteria (HDB) in soil close to the anode increased by nearly two orders of magnitude in the MFC assisted system (373 ± 56 × 103 CFU/g-soil) than that in the disconnected control (8 ± 2 × 103 CFU/g-soil), providing a solid evidence for in situ biostimulation of HDB growth by colonization of EB in the same system. Biotechnol. Bioeng. 2012; 109:426–433. © 2011 Wiley Periodicals, Inc.