To identify hosts that may serve as European reservoirs for the agent of Lyme disease, Borrelia burgdorferi, we determined whether nymphal Ixodes ricinus feed mainly on particular mice (Apodemus flavicollis or A. agrarius), voles (Clethrionomys glareolus) or on sand lizards (Lacerta agilis) and whether the abundance of these hosts corresponds to the seasonal activity of the subadult stages of the vector tick. In all sites, the mice appeared most heavily infested by larvae; at least seven parasitized each mouse, about three per vole and four per lizard. Many fewer nymphal I. ricinus parasitized A. flavicollis and C. glareolus than did larvae. Although more than 30 times as many larval than nymphal ticks parasitized the two most abundant hosts (C. glareolus and A. flavicollis), about 15 times as many fed on A. agrarius and twice as many on lizards. Nymphal and larval ticks fed on rodents at about the same time. Lizards were most abundantly parasitized by nymphs somewhat earlier than by larvae. Early in the season of transmission of Lyme disease, virtually all A. agrarius as well as lizards were potentially exposed to spirochetes borne by nymphal I. ricinus. We concluded that larval and nymphal I. ricinus differentially parasitize different hosts. Because so many of these nymphs feed on them, A. agrarius may more effectively serve as reservoirs for the agent of Lyme disease than do other putative reservoir hosts. The presence of lizards may inhibit transmission.