Faeces samples from 11 sows and their litters and intestinal contents and scrapings collected from 56 piglets at the time of slaughter were examined for Isospora suis. The findings indicate that oocysts with sporoblasts can be found in faeces samples 24 hours after collection; the prepatent period is about six to nine days and the patent period three to eight days (adjudged under non-experimental conditions); piglets in an infected herd are normally resistant to natural infection for the first three weeks of life. Infection is followed by the development of an active immunity which is not affected by the stress of farrowing.