ABSTRACT: Kelps and fucoids are important members of temperate seaweed communities, but may be negatively impacted by climate change and non-native species. We used a field experiment to investigate the effect of higher temperatures and a non-native seaweed, Sargassum muticum, on the kelp Saccharina latissima and fucoid Fucus serratus. All 3 are canopy-forming species which may grow together in the infralittoral and upper sublittoral zones in southwestern Norway. Artificial assemblages with different combinations of the species were placed in the shallow sublittoral, and length changes, weight changes and survival of the thalli were measured. This was done during a hot summer and again during a cool summer. The results showed that the species and their competitive interactions were affected by the different thermal conditions. S. latissima was the most successful species in the cool summer and had an impact on the other 2 species, but it was strongly negatively affected by the hot summer. Under these conditions, F. serratus became the most successful species, gaining the most weight. The effect of Sargassum muticum on the native species was no larger than the effect of intraspecific competition within those species. At the end of both summers S. muticum was in poor condition, potentially caused by low seawater nutrients resulting in low internal nitrogen.