Intestinal microsporidiosis with Enterocytozoon bieneusi was diagnosed in three of 18 HIV-infected patients with chronic diarrhoea. In two cases all known stages of the life cycle of E. bieneusi (merogonial plasmodia, sporogonial plasmodia, sporoblasts, spores) were found in duodenal biopsies by electron microscopical examination, whereas in the third case only merogonial and sporogonial stages were seen. Spores were also visible by light microscopy in semithin sections. Two patients were treated with albendazole (2 x 400 mg/day for 4 weeks) but showed no response. These findings underline the concept of the worldwide distribution of this parasite and verify that it is also frequent in Germany.