Most studies documenting the importance of alien macroalgae relative to native species are based on smaller-scale sampling programmes. Between 1989 and 2003, a Danish monitoring programme collected data on the percentage cover of macroalgae at more than 600 locations throughout the country. We examined this data set to estimate the relative abundance of alien species in the algal flora on large spatiotemporal scales, i.e. across depth ranges, regions and years. Of the 10 alien macroalgal species known to inhabit Danish coastal waters, nine were found in the survey. Most of the alien species were only present in low quantities (<1% of the entire flora). In contrast, the two most common alien species, Sargassum muticum and Bonnemaisonia hamifera, constituted 2-7% of the assemblages, depending on depth, region and year. Sargassum muticum was abundant from 0 to 5 m in the northwestern region, where salinity and species richness are highest, whereas B. hamifera was abundant in several regions in deeper waters, where the native flora is species-poor. Based on their relatively high abundance, we hypothesize that these two aliens have had the largest impact on the native communities. Of some concern is the recent introduction of Gracilaria vermiculophylla. This species has traits that match the conditions of Danish estuaries and may become widespread with potential negative impacts on native biota.