The present invention relates to a process for producing improved strains of transgenic seaweed by genetic manipulation and genetic engineering. The transgenic seaweeds are produced by preparing of seaweed thalli; propagating of plasmid containing the desired genes in a microorganism; isolating and purifying said propagated plasmid; coating of said purified plasmid onto microparticles; introducing said microparticles into the prepared seaweed thalli by particle bombardment; and culturing these to produce transgenic multicellular material. Preferably, said seaweeds are gracilaria, and more preferably gracilaria changii.