Salmonella is a leading cause of foodborne illness in the United States. It is a zoonotic pathogen found in many species of food animals, and contamination of foodstuffs by strains of Salmonella found on farms is an important source of human exposure. Here we describe a long-term (2004-2010) study of Salmonella colonization on a typical dairy farm in the Northeastern United States. The fecal shedding prevalence in the herd ranged from 8% to 97%, and greater than 50% of the herd was shedding Salmonella for more than two-thirds of the study period. Salmonella enterica serotype Cerro was first detected in September 2004, after a small and very short-lived outbreak of Salmonella Kentucky. Cerro persisted within the herd for over 3 years, with no clinical signs of salmonellosis in the animals. In the winter of 2006, Kentucky was again detected within the herd, and over a 2-year period, Kentucky gradually supplanted Cerro. Kentucky was the only serotype detected from March 2008 until September 2009, when Cerro was again detected in 15% of the cows on the farm. Since September 2009, Kentucky and Cerro have coexisted within the herd, which continues to harbor these serotypes at high prevalence. Pulsed-field gel electrophoresis (PFGE) could not discern differences between Cerro strains isolated during this study, but it did suggest that the strain of Kentucky that seemed to behave as a commensal in these dairy cows is distinct from the transient strain isolated in 2004. Understanding the dynamics of competition between these two serotypes that seem to behave as commensal colonizers of dairy cows may provide insights into the mechanisms by which Salmonella establishes infection in the lower gut of dairy cows and may lead to the development of measures to prevent or limit Salmonella colonization of dairy cows.