A total of 32 strains ofYersinia enterocoliticaandY. enterocolitica‐like microbes were examined for their ability to produceE. coliST‐like enterotoxin when grown at 22 °C and 37 °C, using the infant mouse assay. Enterotoxin production was indicated for 19 (59%) of the 32 strains examined. Nine of ten sucrose non‐fermenting strains belonging to O‐serogroup 28 produced enterotoxin at both 22 °C and at 37 °C. Eight of these strains were obtained from the intestinal contents of apparently healthy small mammals or from water samples at three different localities in Norway. One O‐serogroup 28 culture was a reference strain received from Institut Pasteur, Paris. With the exception of O‐serogroup 28, enterotoxins were detected only after incubation at 22 °C. The infant mouse assay was positive for all the four clinical isolates belonging to O‐serogroup 3/biotype 4. Six of 13 different reference strains were posi