2429 samples of drinking water supplies in South Bavaria were examined for Yersinia between October 1982 and March 1983. Yersinia enterocolitica was isolated from 82 of 1650 samples of central water supplies and from 42 of 779 samples of decentralized water supplies. Only three of these strains were serotype O3 but indole-positive and different by that from the indole-negative human pathogenic serotype O3 which is most frequent in Western Europe. No strain was serotype O9. Y.e. was more often isolated from water samples which were objected according to the German drinking water decree to contain Escherichia coli, coliforms or increased colony counts. This points thereupon that Y.e. do not originally belong to the saprophytic soil bacteria but reaches the drinking water subsequently by fecal pollution. As compared with ENDO-Agar, CIN-Agar was found the preferable medium with a three times higher isolation rate. In addition, Y. intermedia and Y. frederiksenii were isolated from 20 and 23 samples respectively.