The effect of cyanobacterial extracellular byproducts upon the algal photosynthesis was assessed to elucidate the allelopathic effect of algal blooms in eutrophic lakes. Algal cultures were filtered with 0.2㎛ membrane filters to remove algal cells and sterilized. 0 to 30㎖ aliquots of filtrate were added to 25㎖ culture suspensions of other algal species, and the ^14C uptake rates were measured. The filtrates of cyanobacterial cultures (Anabaena flos-aquae, Phormidium sp.) inhibited the photosynthesis of green algae (Selenastrum capricornutum and Scenedesmus brasiliensis), while the filtrate of Selenastrum capricornutum and Scenedesmus brasiliensis did not inhibit Anabaena flos-aquae. In the autoinhibition test Anabaena flos-aquae, Selenastrum capricornutum, and Scenedesmus brasiliensis showed positive, negative, and no effects, respectively.