Abstract: Two large-sized enclosures (each 20 ?29m) were set up in the Shuiguo Hu Bay near the west end of the hypereutrophic Lake Donghu. At the beginning of the experiment (October 1995),no macrophytes were present in the enclosures and a submersed macrophyte, Potamogeton crispus, was introduced into both enclosures. In April 1996, P. crispus became very abundant in both enclosures (The maximum 214-381gD. W / m2). Compared with lake water outside the enclosures, the reestablishment of P. crispus significantly decreased the concentrations of various nitrogen and phosphorus compounds and conduchvity, and increased significantly dissolved oxygen and transparency of the water in the enclosures (p0.001). In April 7, P. crispus were removed from Encl. B. One month later, the concentrations of nutrients and chlorophyll a were only slightly higher in Encl. B than in Encl. A, and there was no statistically significant difference between them, i.e., the removal of P. crispus did not cause a significant deterioration of water quality within such a short period of time.