In Thailand in 1999-2000, Cyclospora oocysts from two HIV-infected patients and one patient with prolonged diarrhea were detected by formalin-ether concentration technique. Sporulation was performed by mixing stool samples in 2.5% potassium dichromate solution, sporulated oocysts were then treated with various solutions before mechanical rupturing in order to establish excystation, fewer than 10% of the sporulated oocysts could be excysted. Our techniques provided more details of the characteristic appearance of sporocysts and sporozoites within the oocysts (DMSO-modified acid-fast technique with our modification).