Good evidence has accumulated pointing to enteropathogenic Escherichia coli as a cause of infantile diarrhea. During the course of this one-year bacteriological investigation, Esch. coli were isolated from 321 household pets, 237 dogs and 84 cats that were apparently healthy. Rectal smear specimens were obtained in the study through the cooperation of veterinarians and families. A higher percentage of positive cultures was obtained during the summer months. Epidemiologic study in the case of an 11-month-old infant hospitalized for gastroenteritis revealed that a dog in the household was the specific source of infection.