Objective:To obtain the prevalence with clinical symptoms of Cyclospora cayetanensis(C.cayetanensis),a coceidian protozoan parasite,in Thailand which is the cause of an intestinal infection characterized by sporadic-to-frequent explosive diarrhea.Methods:In a field surveyconducted by the Faculty of Tropical Medicine.Mahidol Lniversity.as part of the existing parasite-control program,a total of 2 540 faecal samples from villagers in Nan Province,Thailand,were collected and examined to determine the prevalence and clinical characteristics of parasitic infections.Results:Twelve cases of C.cayetanensis infection were found duritig faecal examination of schoolchildren aged 5-12 years.None exhibited obvious clinical symptoms,especially evidence of diarrhea:5 of 12 had loose faeces,one reported frequent symptoms of abdominal discomfort,and another had pale conjunctiva with low hematocrit.The children were generally asymptomatic.Conclusions:This finding confirms a public-health issue with potentially serious consequences whereby children can be exposed to an environment contaminated with food-und water-borne transmitted oocysts,anil can hence become infected with C.cayetunensis. Objective: To obtain the prevalence with clinical symptoms of Cyclospora cayetanensis (C. caeetanensis), a coceidian protozoan parasite, in thailand which is the cause of an intestinal infection characterized by sporadic-to-frequent explosive diarrhea. Methods: In a field surveyconducted by the Faculty of Tropical Medicine. Mahidol Lniversity. as part of the existing parasite-control program, a total of 2 540 faecal samples from villagers in Nan Province, Thailand, were collected and examined to determine the prevalence and clinical characteristics of parasitic infections. Results : Twelve cases of C. cayetanensis infection were found duritig faecal examination of schoolchildren aged 5-12 years. None reduced apparent clinical symptoms, especially evidence of diarrhea: 5 of 12 had loose faeces, one reported frequent symptoms of abdominal discomfort, and another had pale conjunctiva with low hematocrit.The children were generally asymptomatic.Conclusions: This finding confirms a public-health issue with pote ntially serious consequences based children can be exposed to an environment contaminated with food-und water-borne transmitted oocysts, anil can become became infected with C. cayetunensis.