Acetic, oxalic, malic, and citric acids significantly inhibited the growth of Colletotrichurm, gloeosporioides, a phytopathogenic fungus, and acetic acid showed the strongest inhibition with no growth at 50 mM. The growth inhibition by these organic acids was closely related with the inhibition of respiration, as tested using three species, C. gloeosporioides, C. coccodes, and C. dermatium. Optimum growth of C. gloeosporioides was observed around pH 6.0. The inhibition of growth by acetic acid accelerated along with a decrease in pH from 6.0 to 4.0, suggesting that the inhibition might be more enhanced by undissociated form of acetic acid. Despite of growth inhibition by acetic acid, the fungus was able to grow in a normal medium when acetic acid was eliminated, implying that the growth inhibition may be resulted from an acetic acid-mediated inhibition of respiration than a structural damage of cell. Catalase activity of the fungus increased in response to 0.1% hydrogen peroxide, but addition of this together with 30 mM acetic acid brought about a decrease in the activity. The fungus which showed no grow at 30 mM acetic acid or 0.5% hydrogen peroxide began to grow after the elimination of these. But the fungus added simultaneously by these two compounds did not grow at all despite the elimination of these. Thus, controlling of Colletotrichum might be developed using acetic acid which is generally less dangerous than chemical reagents.