As every sheep farmer knows, diarrhoea is of common occurrence in animals of all ages in his flock. In lambs it can result in sigificant mortality while in older animals growth rates are reduced and weight loss can occur. Although diarrhoea can be an incidental finding in many sheep diseases, particularly in their terminal stages, only on those conditions are commented on in which diarrhoea is the predominant feature of the clinical syndrome. The subject is discussed below on both an age and seasonal basis and, if an initial assumption is made that lambing takes place at the traditional March/April period, animals will be increasing in age as the seasons progress. Thus the sequence commences in the spring with the young lamb and concludes the following winter with the adult.