The mechanisms underlying biotic resistance against the establishment of invasive species in native macroalgal communities are still poorly understood. In the present study, the impact of epiphytes on growth and recruitment of Sargassum muticum, a Pacific seaweed first found in Ireland in 2001, were investigated in different habitats on the Irish west coast. The epiphytic brown alga Pylaiella littoralis was regularly observed on established S. muticum populations. Field investigations revealed that epiphyte coverage was significantly higher on S. muticum than on native macroalgae, and higher in sheltered than exposed sites. Growth and development of S. muticum was not affected by epiphyte coverage at an exposed site. However, recruitment of S. muticum at the sheltered site was about 90% lower than at the exposed site, most likely as a result from combined stresses caused by dense P. littoralis coverage resulting in weight increase and shading. Shading frequently lead to a 50% reduction in light availability to S. muticum and drag appeared to be causing premature frond fragmentation. These findings add a new perspective to the overall competitiveness of S. muticum in sheltered native habitats. Results suggest that invasion resistance on a local scale depends largely on habitat properties and the composition of resident species.