Abstract Twenty‐seven out of 48 ewes and lambs had died following a profuse watery diarrhoea. Initial treatment for parasitic gastroenteritis and housing animals had not stopped the deaths. Postmortem examination of an affected lamb identified vast numbers of immature rumen fluke and confirmed disease due to acute paramphistomosis. The immature rumen fluke were detected by standard worm washes of the abomasum and small intestine used to detect gastrointestinal worms. Documented cases of acute paramphistomosis affecting sheep are rarely reported across the UK and this is the first recorded case affecting a flock in Wales.