The allelopathic activity of the green alga named Scenedesmus acuminatus against the green algaChlorella vulgaris has been reported. Two algae were isolated, purified and identified from water of earthen fish ponds of Central Laboratory for Aquaculture Research, Abbassa, Sharkia, Egypt. S. acuminatus was cultured in mass culture. Algal mass was harvested at exponential growth phase and dried then extracted by the organic solvents (ethanol or methanol) using a soxhlet apparatus. Allelopathic activity of the crude extracts was investigated with agar-well diffusion and paper-disc assay to demonstrate efficiency of antialgal principles against C. vulgaris. Findings revealed that two crude extracts had an inhibitory effect against C. vulgaris where, large and small inhibition zone caused by Agar-well diffusion assay. Ethanolic crude extract was 23.6 and 51.6 mm at concentrations of 100 and 400 µl respectively. The largest and smallest clear zone caused by methanolic crude extract was 36.6 and 20 mm in favor of the above mention concentrations. On the other hand, the largest and smallest clear zone caused by paper- disc assay for ethanolic crude extract was 26.3 and 17.6 mm and for methanolic crude extract was 15.3 and 24 mm at concentrations of 10 and 40 µl respectively.