The Capillaria hepatica worm is a common parasite of rats and other rodents. The extent to which domestic rodents are responsible for human infection is not known, since it lodges in the liver of children in poor housing conditions. These children suffer many more conspicuous health problems and generally have poor access to health care. Liver biopsy is usually necessary to make the diagnosis. After a case of infection with Capillaria in a five year old Acapulcan child was reported, CIET initiated a study to determine the prevalence of Capillaria in domestic rodents and to discover if rodents are infecting children who live in the household. Rats caught in dwellings in Acapulco were dissected to ascertain the presence of the worm: some 40% turned out to be heavily infected in the community of the index case.