The sensitivity of an important aquatic macrophyte, duckweed (Lemna major), to UV-B radiation was studied under experimental conditions at three different doses designated as no, mild, and severe injury dose by observing visible injury symptoms and estimating levels of chlorophyll, pheophytin, carotenoids, protein, starch, free sugar, and peroxidase activity. Laboratory-grown duckweed plants were exposed to UV-B radiation at 0.4 mW/cm(2) intensity for different time periods. Mild and severe injury were developed at 6.48 and 8.64 J, respectively Peroxidase activity increased at all the exposure levels. Dose dependent decrease in chlorophyll and starch with drastic depletion in protein and free sugar content were observed Pheophytin and carotenoids content increased at no injury level, but decreased at higher exposure level. The results indicate that ambient W-B radiation at the indicated level acts as a physiological stress in Lemna major.