The carpet sea squirt (Didemnum vexillum) is an invasive non-native species of colonial sea squirt that has recently been found in England, Ireland, Wales and at one location in the Clyde, on the west coast of Scotland. The potential ecological and economic impacts of this species are significant. A rapid response eradication programme in Holyhead Harbour is currently underway, showing signs of some success so far. However, it is not known how far this species has spread within the UK, and surveys are underway in England and Wales to determine its distribution. It is crucial to know as soon as possible how far it has spread, as this information will determine what the appropriate response will be - if it is confined to only a few sites, it may be possible to attempt eradication. If it is already more widespread, then containment or control measures will be more appropriate.