In 1972 and 1973 a nationwide outbreak of typhoid fever occurred in Mexico. The responsible strain of Salmonella typhi had a characteristic pattern of phage lysis, resembling the type A pattern, referred to as degraded Vi(A), and was resistant to chloramphenicol and other antimicrobial agents in vitro and in vivo. Eighty cases of infection with strains of S. typhi that were related to the Mexican epidemic strain were reported in the United States. The epidemic in Mexico subsided in mid-1973, and no further cases of typhoid fever due to chloramphenicol-resistant organisms were reported in the United States. Infections with chloramphenicol-sensitive strains of S. typhi with the phage lysis pattern of degraded (Vi(A) occurred in association with travel in Mexico before and after the height of the epidemic in Mexico. Although typhoid fever due to chloramphenicol-resistant organisms has not been reported in the United States since the subsidence of the Mexican epidemic, testing of isolates of S. typhi for antibiotic sensitivity is recommended because of the continued existence of resistant strains elsewhere.