In two studies carried out in 1974-75 on 10 eight and 25 two week old piglets obtained from a SPF-herd the endoparasitic status was determined. Samples of faeces were examined by microscopy which revealed the presence of Isospora suis, a finding that was quite unexpected in a SPF-herd. During days 17-23 pp. the 25 piglets were not found to excrete oocysts, the earliest findings of oocysts being made 24-35 days p.p. The 10 piglets being kept isolated in individual pens excreted oocysts up until 88 days p.p. The epidemiology of Isospora suis is discussed.