Ruminant livestock, particularly cattle, is considered the primary reservoir of Escherichia coli O157:H7. This study examines the transmission of E. coli O157:H7 within groups of cattle during winter housing. Holstein Friesian steers were grouped in six pens of five animals. An animal inoculated with and proven to be shedding a marked strain of E. coli O157: H7 was introduced into each pen. Fecal (rectal swabs) and hide samples (900 cm2 from the right rump) were taken from the 36 animals throughout the study. Water, feed, and gate or partition samples from each pen were also examined. Within 24 h of introducing the inoculated animals into the pens, samples collected from the drinking water, pen barriers, and animal hides were positive for the pathogen. Within 48 h, the hides of 20 (66%) of 30 cohort animals from the six pens were contaminated with E. coli O157:H7. The first positive fecal samples from the noninoculated cohort animals were detected 3 days after the introduction of the inoculated steers. During the 23 days of the study, 15 of 30 cohort animals shed the marked E. coli O157:H7 strain in their feces on at least one occasion. Animal behavior in the pens was monitored during a 12-h period using closed circuit television cameras. The camera footage showed an average of 13 instances of animal grooming in each pen per hour. The study suggests that transmission of E. coli O157:H7 between animals may occur following ingestion of the pathogen at low levels and that animal hide may be an important source of transmission.