Escherichia coli O157:H7 is the principal causative organism of haemorrhagic colitis and haemolytic ureamic syndrome. There have been a number of outbreaks associated with this bacterium and it is considered that it is associated with food-producing animals, although it had not been possible to identify a particular source. During the investigation of an outbreak at a kindergarten in south-west Ontario there was found to be a significant association between infection and the consumption of unpasteurized milk. Faecal samples from healthy cows and calves on the implicated farm were investigated for Esch. coli O157:H7 using MacConkey-sorbitol medium. Seven animals were found to be carrying VTEC and 2 of the isolates were positively identified as O157:H7. The other 5 have yet to be serotyped. The findings provide the first direct evidence that cattle may be the primary reservoir of Esch. coli O157:H7.newline˜T.J. Humphrey