Abstract: With the effects of competition for light and nutrients and the microbial interference ruled out among plants, allelopathic effects of Alternanthera philoxeroides,Trapa spp., Ceratophyllum demersum and Lemna minor on Scenedesmus arcuatus Lemm were examined in comparison with that of Eichhornia crassips .The results showed that the test plants reduced,in varied degree, the population of the algae by stimulating destruction of its chlorophyll a and increasing of its MDA content and inhibiting of its SOD activity.The water in which the plants were cultured also showed similar allelopathic effect.But the allelopathic effect was not as effective as that of Eichhornia crassips .It seems that the allelopathic effect of aquatic higher plants on the algae was somewhat a common phenomenon.

Key words: allelopathy, higher aquatic plant, Scenedesmus arcuatus Lemm