The present invention relates to a process for producing improved strains of transgenic seaweeds via Agrobacterium-mediated transformation. The transgenic seaweeds are produced by preparing of seaweed thalli: culturing of a transformation-competent Agrobacterium species containing desired genes; wounding of multicellular thalli of seaweed by using a line sterilized needle; infecting of wounded seaweed with said transformation-competent Agrobacterium species; and culturing of infected seaweed for a time sufficient to effect transformation of said seaweed. Preferably, said seaweeds arc red seaweeds of genus Gracilaria, and more preferably Gracilaria changii.