In the area, controlled by the Institute of Microbiology of St. Gall, 262 S. typhi murium strains could be isolated during 1973. By means of phage typing, according to GUINEE, 20 different phage types as well as 6 additional atypically reacting strains, with characteristic atypical pattern, were recognized. The phage type 650 was most frequently encountered (37.0%) folowed by types 61 (20.2%) and 505 (10.3%). An intensive change in phage types was noted among 114 cases, observed over 7 months, in the region of Wil SG. In the beginning, 5 different phage types were determined, but in the following 2 months, they were replaced by the type 650, and, finally, by the type 61. Phage typing enabled the understanding of epidemiologic relationship of two outbreaks occurring in Kirchberg (type 650) and Bütschwil (type 61) by consumption of infected slaughtered beef. Also, an outbreak of the infection in Wil, transmitted by an infected butcher, could be cleared. Testing of the resistance against tetracycline, chloramphenicol, ampicillin and trimethoprim-sulphamethoxazol showed that 80.1% of the strains were sensitive and 18.3% were resistent against tetracycline; some phage types were sensitive (2, 3, 4, 61 etc.), predominantly sensitive (650) or resistent (330, 450, 504, 505 ARS-12). Four strains were resistent against one or more chemotherapeutic agents used.