The parasitic copepod, Splanchnotrophus willemi, is recorded infecting the nudibranch Ancula gibbosa for the first time in the UK. Seven out of twenty Ancula gibbosa sampled from Mumbles, Swansea, UK, were found to be infected with S. willemi. Light microscopy was used to investigate the effects of S. willemi on its host. In six hosts the internal damage caused was minimal. However, in one host the damage was extensive resulting in the disintegration of the digestive and reproductive viscera. One other A. gibbosa not parasitized, exhibited similar damage and it was postulated that this animal may have survived infection. In all cases of infection there was never more than one male or one female S. willemi present.