The effect of extracts from brown algae on the growth of five kinds of marine phytoplankton was examined. Ethyl acetate extracts from Arame, Eisenia bicyclis and Ishige, Jshige ohamurae strongly inhibited the growth of plankton. Ethyl acetate extract from Arame almost completely inhibited the growth of five kinds of plankton at a concentration of 50μg/ml. Among the five kinds of plankton used for the experiment, Chaetoceros debile was the most sensitive to both the ethyl acetate extract and the water-soluble materials. Prorocentrum rnicans and Fibrocapsa japonica demonstrated some tolerance to the extract and to the water-soluble materials. Protoplasts were isolated from the thalli of Susabinori, Porphovra yezoensis and Anaaosa, Ulva pertusa, and then they were maintained with PES agarose medium containing Arame extract. The growth of the protoplasts was observed under a microscope. When Arame ethyl acetate extract was added to the medium at a concentration of 50μg/ml, the protoplasts grew normally. However, when the concentration of Arame ethyl acetate extract was increased, the growth rate of the protoplasts decreased. Eventually, the cells failed and ceased to grow.