Bacteriophages are natural predators of bacteria and may mitigate Escherichia coli O157:H7 in cattle and their environment. As bacteriophages targeted to E. coli O157:H7 (phages) lose activity at low pH, protection from gastric acidity may enhance efficacy of orally administered phages. Polymer encapsulation of four phages, wV8, rV5, wV7, and wV11, and exposure to pH 3.0 for 20 min resulted in an average 13.6% recovery of phages after release from encapsulation at pH 7.2. In contrast, untreated phages under similar conditions had a complete loss of activity. Steers (n = 24) received 10(11) CFU of naladixic acid-resistant E. coli O157:H7 on day 0 and were housed in six pens of four steers. Two pens were control (naladixic acid-resistant E. coli O157:H7 only), and the remaining pens received polymer-encapsulated phages (Ephage) on days -1, 1, 3, 6, and 8. Two pens received Ephage orally in gelatin capsules (bolus; 10(10) PFU per steer per day), and the remaining two pens received Ephage top-dressed on their feed (feed; estimated 10(11) PFU per steer per day). Shedding of E. coli O157:H7 was monitored for 10 weeks by collecting fecal grab and hide swab samples. Acceptable activity of mixed phages at delivery to steers was found for bolus and feed, averaging 1.82 and 1.13 x 10(9) PFU/g, respectively. However, Ephage did not reduce shedding of naladixic acid-resistant E. coli O157:H7, although duration of shedding was reduced by 14 days (P < 0.1) in bolus-fed steers as compared with control steers. Two successful systems for delivery of Ephage were developed, but a better understanding of phage-E. coli O157:H7 ecology is required to make phage therapy a viable strategy for mitigation of this organism in feedlot cattle.