As part of a program to control endemic typhoid fever in Santiago, Chile, an assessment was made of the magnitude of the reservoir of chronic carriers of Salmonella typhi. The availability of an accurate census and reliable data on the prevalence of biliary disease and of S. typhi carriage among persons with cholecystitis allowed an unusually precise estimate of the number of carriers. In 1980 there existed 25,019 female and 4,575 male carriers in a population of 4,264,514, yielding a crude prevalence of 694 carriers per 10' population. Because of the magnitude of this human reservoir, which includes many females of <40 years of age, it is recommended that a typhoid control program include the identification of carriers followed by health education and therapeutic interventions. © 1982 by the University of Chicago.