The utility of rDNA internal transcribed spacer (ITS) sequences for separating species of Ulva along the North Brittany coast of France was investigated. Particular attention was paid to the Ulva species responsible for ‘green tides’. Floating thalli are difficult to identify due to fragmentation and it was not clear at the outset whether one or more species of Ulva might be contributing to the green tide phenomenon. Results show that ITS sequences are good markers for Ulva species and that the Lannion Bay green tide taxon is a previously unrecognized species that can also be identified on morphological grounds. We found that ITS type did not necessarily correspond to the initial identification of a specimen based on morphological features, due to the lack of good diagnostic characters. This survey highlights the need for a more intensive survey of Ulva populations along European coasts. Once ITS groups can be recognized, quick screening assays can be developed to aid morphological identification.