A case of acute human isosporiasis in Thailand was reported. A 57-year-old man developed diarrhoea after prednisolone administration for the management of nephrotic syndrome. Stool examination revealed numerous Isospora oocysts. On the basis of parasite morphology and sporulation time, it was identified as Isospora belli Wenyon, 1923. The patient recovered for the diarrhoea without specific treatment.