This study compared seasonal growth, development and reproduction of the invasive brown macroalga Sargassum muticum in habitats with different wave exposure on the Irish west coast. Three field sites with different degrees of wave exposure were chosen for monthly observations to reflect different habitats that were characteristic of the Irish west coast. Growth and receptacle development differed considerably between sites. Growth and receptacle development was lower at the most sheltered site. Here, S. muticum showed signs of early fragmentation in April/May during the two years of investigation (2007 and 2008), whilst the population at an exposed site developed normally and plants grew to a maximum average length of 163 cm by July, with the onset of fragmentation in August. Sargassum muticum in a tide pool exhibited a similar seasonal growth cycle as plants at the exposed open shore site. Overall growth however was stunted, with plants reaching a maximum length of only 30-40 cm in July. Receptacle development was also inhibited at the sheltered site, with a maximum of only 10% of plants found to be fertile during spring and summer 2008, while plants at the exposed site and the tide pool exhibited 100% plant fertility by August. An extensive occurrence of the native epiphyte Pylaiella littoralis on S. muticum was noticed during field sampling at the sheltered study site which may have contributed to inhibited development of S. muticum observed in this area.Seasonal biomass production, photosynthetic activity and plant/frond ratio development were contrasted between Sargassum muticum at the open shore and the tide pool. Sargassum muticum biomass production in the tide pool was 3.5 times lower than that of plants on the open shore. Receptacle development and seasonal photosynthetic activity were similar for tide pool and open shore plants, irrespective of morphological differences. Highest photosynthetic rates (fluorescence yield, Y-o) were measured during active growth in February and lowest values during development of reproductive tissue and senescence. Numbers of S. muticum plants on the open shore decreased significantly during the year, whilst frond number per plant increased, possibly suggesting self-thinning through shading processes. The high variability in Sargassum muticum productivity and reproductive development between habitats suggests that the degree of competition and impact on native flora could be site-specific, and their local effect on displacements will require further investigations. (C) 2010 Elsevier Ltd. All rights reserved.