Escherichia coli O157 is a strain of coliform bacteria associated with serious human disease; infection by the strain, particularly in children and the elderly, may lead to death. Although human infection is often associated with food poisoning, other routes of infection have been identified. These include contact with animals, particularly livestock, bathing in infected water, and mere wandering in rural areas. Aside from the human cost in terms of distress to the afflicted and to their families, there is an economic cost. Although there is extensive literature on E. coli O157, much remains uncertain. The present thesis addresses some of these uncertainties. It specifically addresses livestock (cattle and sheep) infection within North Wales. Issues addressed comprise livestock faecal E. coli and other coliform bacteria counts within the area; the effects of grazing on the said counts, and differences in land type (upland or lowland) on them; seasonal variations; the effects of physical and chemical soil properties on E. coli and other coliform bacteria populations; the differential effects of grazing system and crop type on the bacterial populations; and transmission of E. coli O157 to different types of location by yellow dung flies (Scathophaga stercoraria). Investigation of these issues comprises field studies and, in the case of transmission of the bacteria by dung flies, laboratory experiments. The majority of these studies were successful insofar as they suggest robust conclusions. The most important of these are that sheep faeces host more non-pathogenic E. coli than does cattle faeces, that E. coli populations within the environment vary by season, that lowland farms have higher populations of E. coli, that intensive grazing adds to risk of infection, that yellow dung flies can carry substantial populations of E. coli O157, and that the flies may transmit the bacteria to the environment, particularly to water and to vegetation (grass).