Abstract: In order to mitigate the autotoxicity in growing plants in closed hydroponic systems, the effects of foliar applications of 2,4-dichlorophenoxyacetic acid (2,4-D) and 1-naphthaleneacetic acid (NAA) on the growth of strawberry were investigated. Although the growth of strawberry plantlets was not affected by the auxin treatments in the fresh nutrient solution, the auxin treatments recovered the growth in the used nutrient solution. Benzoic acid, a compound reportedly accumulating in the reused nutrient solution of strawberry hydroponics, resulted in a significant decrease in the growth of strawberry plantlets at 50 uM concentration, compared to the growth in the nutrient solution without benzoic acid. Mitigation of the growth inhibition caused by the previously used nutrient solution or addition of the high concentration of benzoic acid in the fresh solution was demonstrated by immersing strawberry leaves in the auxin solutions (0.45 and 4.5 uM 2,4-D or 5.4 and 54.0 uM NAA) for two seconds before transplanting. The number of flowers and harvested fruits, and the fruit yield of strawberry plants grown in the greenhouse for about 33 weeks were reduced by the non-renewing the nutrient solutions. These values recovered in the 5.4 uM NAA treatment and were not significantly different from the control (renewal of the nutrient solution). These results suggested that reductions in the number of flowers and the yield of strawberry in closed hydroponic systems appear to be related to the allelochemicals exuded by the plant itself. The auxin such as NAA would avoid the growth reduction of strawberry caused by autotoxicity. The 5.4 uM NAA treatment may be the most effective for alleviating autotoxicity of strawberry and increasing the yield.