The allelopathic effects of Alternanthera philoxeroides, Pistia stratiotes, Azolla imbricata, Lemna minor, Spirodela polyrhiza and Nasturtium officinale against algae were examined compared with those of Eichhornia crassipes. The alga, Chlamydomonas reinhardi, was selected as the object of inhibition. Under experimental conditions in which the light competition was eliminated, the former five species all exerted an inhibitory effect on the growth of C. reinhardi. This effect of A. philoxeroides or P. stratiotes was not so strong as that of E. crassipes. In the case of Nasturtium officinale, no inhibition but even a stimulation was observed. The crude extracts of exudate from the water in which A. philoxeroides or P. stratiotes had been cultured were strongly inhibitory to the growth of the C. reinhardi. It seems that the allelopathic effect of higher aquatic plants on algae was a somewhat common phenomenon which might play an important role in the formation and succession of aquatic ecosystem.