The occurrence of hyperhydricity in transgenic potato plants, which carry a chloroplastic Cu/ZnSOD gene of the lily in either a sense or antisense orientation, was investigated during in vitro culturing. Hyperhydric symptoms included curled leaves with a succulent appearance that were mainly observed in Cu/ZnSOD sense plants. By contrast, antisense plants showed a low frequency of hyperhydricity and a high rate of ex vitro survival. Therefore, a knockdown approach for a chCu/ZnSOD gene could be a practical method to reduce hyperhydricity in plant micropropagation.