A cluster-sampling, cross-sectional study was conducted for assessing the prevalence of Cryptosporidium infection in children less than 16 years of age from three villages, Dondian, Linshan, and Fuziyin, in rural Anhui in eastern China. Among 320 apparently healthy children less than 10 years of age from Dondian who had stool specimens collected, cryptosporidial oocysts were found in stools of three children from Dondian, and no positive specimens were found in 239 children studied from Linshan. In addition, a total of 610 serum samples from children in these three villages were tested for specific IgG antibody to Cryptosporidium with an enzyme-linked immunosorbent assay (ELISA) and the prevalence rates were 42.3%, 51.7%, and 57.5%, respectively, in Dondian, Linshan, and Fuziyin. Seroprevalence increased progressively with age. No detectable antibody was found in infants between two and six months of age, and seropositivity steadily increased after one year of age. Among 36 sera from adults 15-60 years of age without diarrheal illness in Huanglu villages of rural Chaohu, 50% (18 of 36) were positive. As expected, a good correlation was found in the specific IgG antibody between the paired serum specimens from 30 matched mother-neonates who showed transplacental transfer of IgG. However, little or no IgM antibody was seen in the neonates even though several mothers had a positive anticryptosporidial IgM enzyme-linked immunoassay result. Forty randomly selected serum samples from children less than four years of age in a similarly impoverished semiurban community in Fortaleza, Brazil, where the majority of households also have pit toilets and shared community water supplies and 172 serum samples from patients one month to 29 years of age admitted to the University of Virginia Hospital without diarrhea were also examined. In Fortaleza, almost all children acquired antibody by their second year of life, demonstrating the high prevalence of this infection. In rural Anhui, only about half the children were infected by 5-7 years of age. The overall prevalence rate (16.9%) of seropositivity among children and young adults in Virginia was much lower than in China and Brazil. These results indicate that cryptosporidial infection is ubiquitous, and is highly endemic in these impoverished communities. The difference between China and Brazil may reflect earlier weaning, hygiene practices, poorer water or sanitation, multiple siblings in family and geographic environment in Brazil.