The allelochemical benzoic acid, released from plant roots, is an important inhibitor of plant growth. It was found that Aspergillus flavus Z5 isolated from flue-cured tobacco root reduced the damage of tobacco seedlings caused by root exudates. Results indicated that A. flavus had a positive effect on root morphological characteristics, chlorophyll content, and photosynthetic characteristics. A. flavus grown on Martun medium with benzoic acid as the sole carbon source in a wide range of concentrations. The optimum growth concentration for this strain was 800 mu mol/l, and the highest degradation rate was 2.833 mg/d between 7 and 9 days of fungal culture.