The roles of indigenous and augmented microorganisms in the process of biodegradation of spilled diesel fuel (~2 years old contamination, ~2500 C₁₀-C₄₀ mg/kg initial concentration) were assessed in ~2-month pot experiment as supporting information for selection of an applicable bioremediation method. Non-sterile soil and semi-sterile autoclaved soil were inoculated with Comamonas acidovorans strain with high biodegradation ability and mixed-culture extracted from the soil and enriched on mineral medium with diesel as a sole source of carbon and energy. Significant decrease of C₁₀-C₄₀ concentration was observed in all variants except in the autoclaved control. Analyses of variance revealed that only the “augmentation” factor contributed to significantly more effective biodegradation, while factors “soil sterilization” and “inoculum type” were insignificant. With reference to microbial biomass, the results suggested that the soil was insufficient in number of microorganisms capable of petrol biodegradation and indicated that bioaugmentation was the important treatment in order to enhance bioremediation process.