Species of Ulva occur worldwide in all aquatic habitats from freshwater through brackish to fully saline environments. The scarcity of stable morphological characters and the morphological plasticity linked to environmental conditions typical of species referred to this genus make identification and circumscription problematic. In Ireland and Britain the taxonomy of Ulva species has been uncertain, and only three species have been reported: Ulva lactuca, U. rigida and U. olivascens (later reassigned to Umbraulva olivascens). We reassessed the taxonomic status of this genus by combining morphological investigations with analyses of rbcL sequence data for 24 selected specimens from the Irish and southern British shores. This combination of methods allowed the identification of seven lineages, which were considered to represent the following species: Ulva rigida, U. scandinavica, U. lactuca, U. gigantea, U rotundata, U. californica and Umbraulva olivascens. Of these, U. scandinavica, U. gigantea, U. rotundata and U. californica are newly reported; the distribution of U. olivascens was shown to be much wider than previously thought. The use of morphological features proved to be largely inconclusive and of limited value for circumscription of species. In the rbcL phylogeny, U. olivascens formed a sister clade to all other species. The results support the conspecificity of U. rigida, U. scandinavica and U. armoricana and the conspecificity of U. pseudocurvata and U. compressa already suggested in previous studies.