Eperythrozoon suis infection was identified in a pig herd during an investigation into anaemia and low viability in newborn piglets and severe regenerative macrocytic anaemia in older piglets. The organisms were identified in the erythrocytes of piglets a few days old. Extensive investigations failed to detect other causes of the anaemia and low viability. There was no response to parenteral iron administration alone but the piglets' viability and anaemia responded to the administration of tetracyclines. This is the first report of E suis infection in Northern Ireland.