Type B tularemia caused by Francisella tularensis subsp. holarctica was diagnosed in deer mice (Peromyscus maniculatus) found dead at four sites in west-central Saskatchewan during April and May 2005. The occurrence of tularemia coincided with a decline in the number of deer mice in part of a large area (>22000 km² ) in which deer mice had been extremely abundant during the autumn of 2004 and spring of 2005, and in which mice caused damage to crops in the autumn of 2004. This is apparently the first report of tularemia as a cause of death of wild deer mice. The bacterium isolated from deer mice was atypical in that cysteine was not required in the media used for isolation. Three isolates tested were genotypes not previously identified in Canada. There were no reports of human disease in the area.