Infection with the microsporidian parasite Enterocytozoon bieneusi may be a major cause of prolonged diarrhoea in individuals also infected with human immunodeficiency virus (HIV). The parasite has been reported from Europe, Australia and the Americas, with a prevalence of 7-29%. Faecal specimens were obtained from 202 adults and 106 children in Harare, Zimbabwe, all of whom were in hospital and had diarrhoea. HIV serology was available for 119 adults: 106 were HIV seropositive. There were clinical grounds for suspecting HIV infection in 23 of the remaining patients. E. bieneusi was identified in specimens from 13/129 patients (10%) for whom HIV infection was indicated by serology and/or clinical signs, 1/60 patients (2%) of uncertain HIV status, and 0/13 seronegative patients. 18/106 children were HIV seropositive and 12 were not; HIV serology was not available for the remainder, but 19 were strongly suspected of being infected with HIV on clinical criteria. E. bieneusi was not detected in samples from any child. As is common in Zimbabwe, the prevalence of other parasites in faecal specimens was low and, amongst patients with proven or suspected HIV infection, E. bieneusi was the most prevalent parasite identified, particularly in patients with diarrhoea of over 4 weeks duration.