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readily than microscopical examination of processed tissue,6 but this has not been systematically tested. These five cases ofmicrosporidia represent 6 5% of the African HIV positive patients with intestinal symptoms that we saw. Three of the patients also had cryptosporidiosis (diagnosed on faecal smears, though not seen on histological examination). Irrespective of the degree of inflammation, no microsporidia have been identified in 36 small bowel biopsy specimens from HIV negative Ugandan and Zambian controls; no microsporidia have been seen in over 50 rectal biopsy specimens from HIV negative or HIV positive patients. More intestinal biopsy specimens from patients with AIDS should be examined for microsporidia. This group of parasites infects most phyla of invertebrates and all classes of vertebrates; yet E bieneusi has only been described in human enterocytes in association with HIV-1 infection.' It is unresolved whether they are genuine pathogens rather than mere passengers. The enterocytes containing microsporidia in our material and that of other workers do not show obvious damage by light or electron microscopy. The aetiology of the diarrhoea in AIDS is multifactorial, and microsporidia may be involved, possibly by affecting secretion.