In this paper we quantify the interactive effects of wave exposure and intraspecific density on the survivorship and growth of Sargassum muticum in northern Spain. Juveniles settled onto slate plates were experimentally thinned to two densities and transplanted to exposed and sheltered shores. Initially, plants grew faster at the exposed sites but final lengths were similar between exposures after 5 months, largely because of breakages at the top of the main primary lateral of plants at exposed sites. Mean weights of plants did not significantly differ between exposures. The results indicate that S. muticum juveniles growing at high density were longer and thinner but similar in weight to those at low density. Patterns of growth of larger plants transplanted between sheltered sites similarly suggested changes in morphology of S. muticum in response to differences in density. Survivorship of juveniles did not differ between exposures but was significantly poorer at the higher density irrespective of wave exposure. The results of this and previous studies suggest that the abundance of S. muticum in the low intertidal area of exposed shores in northern Spain is limited at several life-history stages. The relative importance of these potential bottlenecks compared with the availability of propagules remains unknown.