Most experimental studies of diversity effects on invasibility have reported negative relationships while observational studies have often found positive correlations between the numbers of exotic and native taxa. Nearly all of these studies have been done with terrestrial plants or aquatic invertebrates. We investigated effects of native macroalgal diversity on invasion success of the introduced macroalga Sargassum muticum (Yendo) Fensholt (Phaeophyceae: Fucales) on the west coast of Vancouver Island. We conducted both observational field surveys of the correlation between native diversity and exotic cover, and experimental manipulations of native diversity in constructed 25×25 cm communities. Field surveys found higher cover of S. muticum in plots with low native diversity, suggesting a negative relationship between diversity and invasibility at the neighbourhood scale. The experiment found initial cover of S. muticum germlings was highest in plots with greater diversity. Over the duration of the experiment cover of settled germlings increased fastest in the low diversity plots, so that there was a weak negative effect of diversity on final cover of the invader after 77 days. The slope of the relationship reversed over time, with field patterns and experimental results converging at the end of the experiment. Our results suggest native diversity has contrasting effects on different stages of invasion. Diversity facilitates invader recruitment of S. muticum but decreases growth and or survivorship.