The present study constitutes the first contribution to the knowledge of Ixodes pararicinus dynamics on mice populations. Sigmodontin mice were trapped for tick collection in spring 2000, summer and fall 2001 in three environmental settings at Parque Nacional El Rey (2415S 6440W). A total of 203 mice of the genera Akodon, Calomys, Oligoryzomys, Oryzomys, and Oxymycterus (in order of abundance) were captured. Fifty-eight (58) of them were infested with 164 larvae and 62 nymphs of ixodids. The genus Calomys was significantly more infested than Akodon (P<0.0001). The predominant tick species was I. pararicinus (139 larvae and 56 nymphs). Larvae prevailed in summer and fall, and nymphs in spring. The level of infestation was higher in adult mice, compared with juvenile and young mice (P<0.001). Other species found were Ixodes loricatus, Ixodes longiscutatus, Ixodes sp., and Amblyomma sp. The results indicate that sigmodontin mice are of importance for the maintenance of I. pararicinus at the study site.