For crowded stands of terrestrial plants, ranging from mosses to trees, plant (or ramet, for clonal plants) density is negatively related to stand biomass. Stand biomass and ramet density were determined for Mazzaella cornucopiae and for Pterocladiella capillacea, two morphologically distinct intertidal clonal red seaweeds, to compare them with terrestrial plants. For these seaweeds, ramet densities were similar to the highest values reported for terrestrial plants (mosses, specifically). Stand biomass was higher than average values expected from the terrestrial interspecific biomass–density relationship, but lower than the limits expected from the terrestrial ultimate biomass–density line. These seaweeds show unexpectedly low ramet slenderness and high biomass packing per unit of volume, compared with the trend observed for terrestrial plants. Possible explanations for these differences are related to the particular physiology and habitat of intertidal clonal seaweeds.