The allocation of biomass to receptacle, holdfast and vegetative tissues differed significantly between Sargassum elegans and Sargassum incisifolium, two sympatric dominant species of the upper intertidal at Reunion Rocks, KwaZulu-Natal, South Africa. Sargassum elegans, a clumped (many axes per holdfast) species with short primary laterals and a prostrate holdfast allocated a greater proportion of its biomass to the holdfast than did S. incisifolium, an erect species with longer primary laterals, fewer main axes and a conical holdfast. Conversely, S. incisifolium allocated more biomass to the production of receptacle tissue than did S. elegans. These different biomass allocation patterns have previously been considered indicative of different reproductive strategies, i.e. that of sexual reproduction vs. that of persistence and spread in a high wave action environment. At any one time, a low percentage (< 15%) of the main axes of S. elegans thalli were found to be reproductive, whereas a significantly higher percentage (> 50 %) of main axes of S. incisifolium were found to be reproductive. Per main axis, S. elegans has a greater reproductive effort than S. incisifolium, however at the thallus level, the opposite was observed. It is suggested that the main axes (modules of the genet) of S. elegans function co-operatively to promote sexual reproduction, but minimize any associated costs.