Malformins, a small family of cyclic pentapeptides, aye active plant growth regulators isolated from the fungus Aspergillus niger. We purified malformin $A_1$ from the crude malformin A minure, and studied its action in the gravitropir response of maize roots. Intact primary roots that had been pretreated vertically with malformin $A_1$ were placed in a humidified box in the horizontal position. Positive curvature (downward) was inhibited in the pretreated roots compared with the control. In addition, we measured the lateral transport of IAA in primary roots. Roots pretreated with malformin $A_1$ did not show asymmetric distribution of IAA between the upper and lower sides of the elongation zone. Malformin $A_1$ also stimulated ethylene production in maize root segments. Our results had suggested that malformin $A_1$ might inhibit the lateral transport of IAA across the roots from the upper to the lower side because of an increased level of ethylene. Therefore, we placed more IAA on the upper side at the initial phase of gravistimulation. These results were consistent with malformin $A_1$ -pretreated roots showing inhibited positive gravitropic curvature.