By using different staining techniques, 479 stool specimens from 212 diarrheic patients with AIDS were examined for microsporidian spores. Calcofluor fluorescence staining of 119 specimens revealed fluorescent ovoid structures of microsporidian size. Staining of these samples according to the method of Weber et al. (R. Weber, R. T. Bryan, R. L. Owen, C. M. Wilcox, L. Gorelkin, and G. S. Visvesvara, N. Engl. J. Med. 326:161-166, 1992) with trichrome produced six specimens with pinkish spores containing the characteristic microsporidian belt-like structure. The 6 specimens were processed for transmission electron microscopy, as were another 21 specimens which did not present the belt-like structure after trichrome staining but which looked highly suspicious after fluorescence staining. In these 21 samples, only fungal spores and, particularly, bacterial Clostridium spores were demonstrated, whereas in the 6 samples diagnosed positive after trichrome staining, the existence of microsporidia could be verified by electron microscopy. Based on our observations, we propose that the belt-like structure seen with the Weber stains in microsporidian spores corresponds to structures existing in priming-stage spores. The results suggest that routine microscopical fecal diagnosis for microsporidian infection should include a screening by fluorescence staining and, subsequently, a confirmatory viewing of fluorescence-positive samples after trichrome staining.