Aims: This study sought to examine the risk posed by house mice transmitting pathogens to livestock on typical mixed-agriculture farms in the UK. Methods and Results: In a 10-month longitudinal study at one farm, 222 faecal samples were taken from mice and 57 swabs from the farm environment; 3.2% and 15.8%, respectively, were positive for Yersinia. Seventy-five intestinal samples were taken from house mice from three other farms and 9.3% were positive for Yersinia. The commonest species was Y. enterocolitica (of a wide range of serotypes); all isolates were non-pathogenic, except one of Y. pseudotuberculosis. Salmonella was not isolated from any sample. Conclusions: This study provides additional evidence that house mice are generally not significant vectors of either pathogenic Yersinia strains or Salmonella species. Significance and Impact of the Study: This is the first longitudinal study of Yersinia in any small mammal population, and shows infection to be a dynamic series of generally non-pathogenic, transient infections.