The frequency of colonization by Clostridium difficile in 273 hospitalized children under 15 years of age was studied. Feces were collected from patients attending the infectious disease service at the Pediatric Hospital IMSS, during a period of 11 months. No colonization was detected in 16 neonates; whereas 10 of 103 children (9.7%) under one year of age, 7 of 84 children (8.3%) from one to five years and 3 of 70 children (4.2%) from five to 15 years of age were colonized. The use of antibiotics and the nutritional state were studied as possible risk factors for colonization. The frequency of colonization was not influenced by the nutritional state, whereas the treatment with antibiotics decreased significantly the colonization in children under one year of age but not in those children over one year of age. In children under one year of age, the cytotoxin was more frequent in cases of diarrhea, and in those over one year no association was found. The 50 strains isolated from these children were classified according to: toxigenicity, sensitivity to antibiotics, phages and bacteriocins. Strains acquired before hospitalization were more toxigenic than those acquired intrahospital. Twelve resistotypes were detected; one of them (V) was more frequent in intrahospital strains. Ten phagobacteriocin types were found, and two of then (D and I) were present only in intrahospital strains. Using this classification scheme, it was found that eight patients were colonized with two different strains at the same time.