This investigation describes some of the husbandry factors influencing the occurrence of colibacillosis in calves. Diarrhoea and mortality were usually associated with an increase in the proportion of "pathogenic" to total E coli to about 50 per cent in the faeces although there were occasions when the proportion of "pathogenic" E coli increased but no disease occurred. These increases often followed changes of diet which appeared to be more important than environmental conditions. The use of uncontaminated houses to break the cycle of infection appeared to have the greatest influence on the disease pattern.