Abstract The morphology of Isospora oöcysts was studied from 6 cases of human infection. There was a considerable variation in the size, shape and the state of development of the oöcysts in freshly passed stools. 80–90% of the oöcysts completed sporulation in 24 hours in potassium bichromate solution. The sporulated oöcysts showed varying degrees of disintegration of the oöcyst wall. In some cases the wall was partially collapsed; in others it was completely collapsed leaving just 2 sporocysts attached to each other. In view of these findings I suggest that I. hominis be regarded as synonymous with I. belli.