Technical Abstract: Direct fed microbials (DFM) have been identified as potential pre-harvest interventions for the reduction of foodborne bacterial pathogens such as E. coli O157:H7. This study evaluated the efficacy of a direct fed microbial (DFM) consisting of Bacillus subtilis strain 166 as an antimicrobial intervention strategy for the reduction of prevalence and load of Escherichia coli O157:H7 in feces and on hides of feedlot cattle. Cattle (n=526) were divided among 16 feedlot pens. Half of the pens received the DFM and the other eight pens did not. Hide and fecal samples were collected from each animal on days 28, 63, and 84 of the feeding trail. Over the course of the 84-day feeding period, there were no significant differences observed between treatments for either hide or fecal prevalence of E. coli O157:H7 or for the percentage of animals that were shedding E. coli O157:H7 at high levels (greater than 200 CFU/g) in their feces or harboring E. coli O157:H7 at high levels (greater than 40 CFU/cm2) on their hides. Also, there was no significant difference between the average daily gains for the treated and control groups with both groups averaging 1.3 kg per day. We concluded that the DFM tested would not be an effective pre-harvest intervention against E. coli O157:H7.