A 27 month epidemiological survey of cryptosporidiosis in stools was conducted in 288 Gabonese children aged between 0 and 2 years (mean 10.2 months). By at least one of two staining techniques, acid-fast and direct immunofluorescence, the rate of infection was determined to be 24%. The rate was 28% in cases of acute diarrhea, in striking contrast with the low prevalence of 14.8% in uninjured gastroenteritus subjects (p < 0.05). The notion of asymptomatic carriers was clearly demonstrated. The maximum of 34.4% was observed for infants aged between 6 and 12 months (p < 0.02). In cases of malnutrition, this rate increased to 31.8%, whereas it was 16.8% in eutrophic children (p < 0.01). For the rainy and dry seasons, it was 31.7% and 19.4%, respectively (p < 0.02). Water is emerging as an important vehicle for the transmission and spread of Cryptosporidium sp. in tropical areas.