Attached Sargassum muticum (Yendo) Fensholt was first observed in the westernmost part of Limfjorden (Denmark) in 1984, and it has since colonized Limfjorden from west to east at a rate of 15 to 17 km yr^-1. By 1997, S. muticum had become the most dominant macroalga, covering more than 6% of the bottom between 0 and 6 m depth, corresponding to ca 35% cover of the hard substrate. In 1990, the cover of S. muticum was strongly correlated with distance from the original source area in the westernmost part of the estuary, suggesting that colonization was still in progress. By 1997, the cover of S. muticum was less correlated with distance from the original source area and more strongly correlated with the amount of hard substrate, indicating that colonization was reaching its climax. The absence of S. muticum from certain parts of Limfjorden by 1997 is therefore best explained by lack of hard substrate in these areas, and not by insufficient colonization time. The increased abundance of S. muticum between 1990 and 1997 affected species richness and diversity of the macroalgal community only marginally. However, multivariate community analysis revealed significant changes in the macroalgal community structure that were closely related to the increased abundance of S. muticum. Not only did the dominance of S. muticum increase significantly from 1990 to 1997, but the cover of several indigenous species belonging to the genera Laminaria, Fucus, and Codium decreased during the same period, indicating competitive displacement through competition for hard substrate and light.