Sessile organisms inhabiting the intertidal zones of rocky shores are exposed to constantly fluctuating and dramatic environmental changes on a daily basis. During low tide, they may be exposed to desiccation, hyper-or hypo-osmotic shock, extreme temperatures or prolonged light exposure, depending on season and latitude. The frequency and duration of these stresses increases with tidal elevation. Desiccation is the most important stress factor, and seaweeds that live in these zones are well adapted to significant water loss, displaying full physiological recovery during rehydration. Although the mechanisms of environmental tolerance in intertidal seaweeds are not completely understood, it has been suggested that reactive oxygen metabolism may play a key role. , a typical middle or high-intertidal species originally found in the south of China, is widely cultivated along the coasts of Fujian and Zhejiang Provinces. Owing to its high desiccation tolerance, simple morphology and ease of culture in the laboratory, is an ideal material for environmental adaption studies. In this study, we observed the physiological responses of blades of during desiccation stress. Reactive oxygen species (ROS), including superoxide anion free radical (, reduced glutathione (GSH) and ascorbate (ASA) content, were measured, as were the activities of superoxide dismutase (SOD), catalase (CAT), glutathione reductase (GR) and ascorbate peroxidase (APX). The results show that an important boundary was reached at 60% water loss from the blades. When water loss was below 60%, the major constituent of ROS produced in the , which has a lower relative toxicity than the more toxic , which did not change significantly. The activity of APX and levels of ASA and GSH did not change significantly, while the activities of SOD, CAT and GR were significantly down-regulated. When water loss was below 60%, as the H₂O₂ content decreased, the levels of the more highly toxic were significantly increased and it became the major constituent of ROS in cells. Under these conditions, the activities of SOD, CAT and APX were still not significantly increased, while the activity of GR and levels of ASA and GSH were significantly up-regulated. These results suggest that GR, ASA and GSH play important roles in removing ROS during serious desiccation stress, while the antioxidant enzymes SOD, CAT and APX are not involved.