We determined the capacity of bank voles (Clethrionomys glareolus) to infect feeding Ixodes ricinus ticks with Borrelia burgdorferi sensu lato (infectivity), during June to October 1991 and June to September 1992 in south-central Sweden. In both years, the infectivity of older voles to ticks was higher in August to September (48% to 59%) than in June to July (20% to 32%). We propose that the infectivity of bank vole populations in Sweden decreases during winter and spring due to death of highly infective voles and recruitment of uninfective young ones, and that the tick vector, rather than the mammalian host, is the primary overwintering reservoir of B. burgdorferi.