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Many intertidal rocky habitats of the Chilean coast were dominated by the dark green thalli of a mat-forming Codium. This green seaweed was ecologically important because it formed a permanent belt by coalescence between conspecific thalli, and thus it affected substrate dominance and community structure. Six species of mat-forming Codium have been recorded in Chile; the adherent Codium in central and northern Chile has been recognized as C. dimorphum, a species first described from the Guaitecas Islands in southern Chile. We analyzed 320 Codium specimens collected from 11 sites spanning the entire Chilean coastline. Morphological and rbcL sequence data revealed the presence of two distinct species with non-overlapping geographical distributions. Codium dimorphum, which is characterized by dimorphic utricles (inner utricles having thin apical walls, and outer utricles having thick apical walls), was found to be restricted to the southern Chilean coast. Specimens from central and northern Chile with uniform utricles were described as a new species, C. bernabei. [ABSTRACT FROM AUTHOR]