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Salmonellae are a major group of foodborne pathogens known to affect both humans and animals. Dairy cattle are a known reservoir of these bacteria and human Salmonella infections have been associated with the consumption of improperly processed or contaminated dairy products. Many of the over 2500 known serotypes of Salmonella are known to infect cattle, resulting in asymptomatic to fatal salmonellosis. This study describes the course of a Salmonella outbreak and subsequent endemic infection on a dairy farm in Pennsylvania. The outbreak was initially detected when a few cows with clinical symptoms and one fatality were found to be infected with Salmonella enterica subsp. enterica serovar Typhimurium var. Copenhagan. Based upon sampling of the farm environment, Salmonella Typhimurium var. Copenhagan was succeeded within 3 months by Salmonella enterica subsp. enterica serovar Kentucky. Salmonella enterica subsp. enterica serovar Cerro ultimately supplanted Typhimurium var. Copenhagan and Kentucky in individual animals and environmental samples and persisted in the herd at high prevalence for almost 2 years. Since there were no obvious clinical consequences of the Salmonella Cerro infection, these data suggest that some serotypes of S. enterica subsp. enterica can behave as commensal organisms in dairy cattle and illustrate the difficulties of controlling Salmonella in milk production systems. The consistent finding of Salmonella in the environment reinforces the potential for human exposure to this pathogen and the need to understand the dynamics and ecology of Salmonella in dairy production settings.