We used a modified trichrome stain to establish the prevalence of microsporidiosis among stool samples taken from HIV-positive subjects in Lusaka with chronic diarrhea for at least 3 weeks. Unconcentrated stool samples from 69 Zambian adults were coded by one of us and sent for blinded analysis by 2 other investigators. Two smears were prepared from each sample using 10 fjL of stool diluted in^(vol/vol) formalin at a ratio of 1: 3 and 1: 5 (vol/vol), fixed in methanol, and stained with a modified trichrome stain as described [4]. Slides were examined independently, with each slide viewed for a minimum of 15 min. A smear was declared positive only if both investigators identified spores in at least one of the two slides when the code was broken. Microsporidia in small numbers were frequently identified only after prolonged microscopy. Final analysis indicated that microsporidia were present in the stools of 16 (23^) of 69 patients.