Acid-fast, coccidian-like bodies (Cyclospora cayetanensis) were identified over the last 18 months in the stools of seven Peruvian patients suffering from diarrhoea. The follow-up of two patients revealed a watery, self-limited diarrhoea, which lasted for up to four weeks. The organism was simultaneously identified in the diarrhoeal stools of three members of the same family who drank unchlorinated canal water and in the stools of a duck bred by this family. The organism was not found in the faecal samples of 50 healthy subjects and 10 ducks bred by families without known diarrhoea cases. These findings, albeit preliminary, may be suggesting that besides consumption of untreated water, additional modes of transmission such as contact with domestic animals may be important in this disease. Further studies are needed to assess whether this disease behaves as a zoonotic condition and to ascertain the relative importance of symptom-free subjects in person-to-person transmission of the organism.