The recent discovery of self-resolving Cryptosporidium infection in immunocompetent patients has aroused growing interest in this parasite, which has thus far been known to affect mainly immunodeficient individuals. Following the hospitalization of two children attending the same day-care center in February 1986 Cryptosporidium oocysts were found in the stools of one of them. This led to an epidemiological investigation to assess the frequency of Cryptosporidium oocysts in the stools of children attending the 7 day-care centers in the city of Poitiers, France. Testing for Cryptosporidium oocysts was performed on formalinized stools, after smear staining by the modified Ziehl-Neelsen procedure. Each positive stool was also submitted to virological and bacteriological examination. Stool specimens from 235 children were studied: 9 (3.8%) of them had Cryptosporidium oocysts in their stools. Four of the children had diarrhea, and 5 (2.2%) were asymptomatic. The existence of healthy carriers should lead to caution in the interpretation of stool parasitology.