Technical Abstract: Listeria monocytogenes, the causative agent of epidemic and sporadic listeriosis, is routinely isolated from many sources, including cattle, yet information on the prevalence of Listeria in beef processing plants in the United States is minimal. From July 2005 through April 2006, four commercial cow/bull processing plants were sampled in the United States to determine the prevalence of Listeria and the serovar diversity of L. monocytogenes. Samples were collected during the summer, fall, winter and spring seasons. Listeria prevalence on hides was consistently higher during cooler seasons. The data collected from pre-evisceration carcass samples mimic the trend found for the prevalence of Listeria on hides. The prevalence on post-intervention carcasses in the chill cooler was below 6%, with the exception of summer and spring samples from one plant where >19% of the carcasses were positive. On hides, serovar 1/2a was the predominant serovar observed, with serovars 1/2b and 4b present 2.5 times less and serovar 1/2c was not detected on any hides sampled. In summary, this prevalence study demonstrates that Listeria species are more prevalent on hides during the winter and spring and that interventions being used in cow/bull processing plants appear to be effective in reducing or eliminating contamination of carcasses with Listeria species.