Master of Science / Department of Animal Sciences and Industry / Evan C. Titgemeyer / Seven ruminally cannulated heifers approximately 225 kg initial BW were used in a 7 × 4 Youden square design to determine the effects of different levels of limonene on ruminal Fusobacterium necrophorum populations. Treatments included: 1) control, 2) limonene at10 mg/kg diet DM, 3) limonene at 20 mg/kg diet DM, 4) limonene at 40 mg/kg diet DM, 5) limonene at 80 mg/kg diet DM, 6) CRINA-L (a blend of essential oil components) at 180 mg/kg diet DM, 7) tylosin at 12 mg/kg diet DM. Each period included 11 d with 10 d washouts between periods. Samples were collected on d 0 (before treatment initiation), 4, 7, and 10 for measuring F. necrophorum by most probable number (MPN) method using selective culture medium. Results indicate that CRINA-L (P = 0.52) and tylosin (P = 0.19) did not affect ruminal F. necrophorum populations. Limonene linearly decreased (P = 0.03) F. necrophorum populations, and the optimal dietary concentration for limonene was 40 mg/kg DM. Limonene did not affect ruminal degradation rate of lysine, NH3 concentration, or VFA profiles in ruminal fluid. Limonene was useful for reducing ruminal concentrations of F. necrophorum. It may have potential to control liver abscesses, although further research will be needed to assess the effect of limonene under feedlot conditions.