A great diversity of VTECs exist but only in the case of Escherichia coli O157:H7, a common human foodborne pathogen, has sufficient research been done to allow generalizations about the ecology. The key features are as follows: lack of host specificity such that indistinguishable isolates can be found in a variety of species; near-ubiquitous distribution in cattle (and perhaps other ruminant) farms; transient residence in the gastrointestinal flora of individual animals that is not associated with clinical disease; temporal clustering at the population level such that most fecal shedding is confined to sharp bursts in a high percentage of animals separated by much longer periods of very low prevalence; a higher prevalence in young animals in comparison to older ones: a higher prevalence in animals with floral disturbance such as that caused by transit, feed changes or antimicrobial dosing; and a markedly higher prevalence during warm months. Molecular epidemiological studies of E. coli O157:H7 have demonstrated that subtypes of the organism can persist on cattle farms for years, thus supporting a conclusion that cattle farms represent a reservoir. Yet on such farms, common subtypes are often found in environmental niches and in other species of animals; thus, it is not completely clear that cattle themselves are the reservoir. New subtypes are periodically observed on particular farms, and indistinguishable subtypes can be found on farms that are separated by hundreds of kilometers even in the absence of any obvious animal movements between them. The number of subtypes found on a farm does not appear to be qualitatively correlated with cattle movements (e.g., purchases) into the farm. Commercial feeds are sometimes contaminated with E. coli O157:H7, and it seems likely that feeds represent an important route of dissemination for this agent and other VTEC. Mixed feeds collected from feeding troughs are commonly positive for E. coli O157:H7, as are cattle watering troughs, and feed and water likely represent the most common means of infection. Environmental replication in feeds and in sediments of watering troughs occurs and may account for the higher level of fecal shedding in the warm months. Since E. coli O157:H7 has been found to persist and remain infective for at least 6 months in water trough sediments, this may be an important environmental niche where the organism survives during periods when it cannot be detected in cattle, especially during cold months. Traditional means of controlling infectious agents, such as eradication or test and removal of carrier animals, do not appear to be feasible for VTECs. Nevertheless, certain farm management practices-especially those related to maintenance and multiplication of the agent in feed and water-may provide practical means to substantially reduce the prevalence of these agents in cattle on farms and in those arriving at slaughter plants.