Technical Abstract: To understand the dynamics of transmission of shiga toxin-producing Escherichia coli O157:H7 (STEC O157) in beef calves, serum samples were obtained from calves in a beef cow-calf herd approximately every 6 weeks from birth until weaning for 3 consecutive years. The presence of specific anti-O157 antibodies in these serum samples was detected using a blocking ELISA assay incorporating an anti-O157 monoclonal antibody. Using seroconversion data, the basic reproduction ratio (Ro) was estimated for each of the 3 years as well as in aggregate using both deterministic and Martingale methods. Ro for STEC O157 infection in range beef calves by deterministic methods varied from 2.9 - 5.6, with an average of 4.3, CI95% 2.8 - 5.9. Martingale estimates of Ro ranged from 3.5 - 7.4, or 5.3, CI95% 3.9 - 6.6, for data from all three years. Given the above estimate of Ro, it is predicted that 65 - 86% of a herd of calves must be effectively vaccinated, or must be rendered non-suceptible through other means, to eliminate STEC O157 infection from a herd.