Abstract: Methods for producing hybrid and transgenic strains of marine algae, and seaweeds and phycocolloids produced by the product seaweeds. Hybrid marine algae are produced by preparing protoplasts from spores of parental algal plants, fusing the spore-protoplast to form heterokaryon fusion products, isolating selected fusion products, and culturing these to produce multicellular material. Similarly, transgenic marine algae are produced by preparing protoplasts from spores of a marine algal plant, introducing foreign DNA into the resulting spore-protoplasts to form transformed spore-protoplasts, selecting certain transformed spore-protoplasts, and culturing these to produce transgenic multicellular material.