We examined the effect of two kinds of barriers on an expanding gray wolf, Canis lupus L., 1758, population in an agricultural habitat in north-central Spain. The barriers were (i) a four-lane fenced highway along a flat area without wildlife-crossing facilities, and (ii) the River Duero artery (RDA), comprising the river itself (50-100 m wide) and several small infraestructures along it. From March 1997 to October 2001, all 4 radio-collared wolves loving <15km from the highway (1 adult territorial male, 1 territorial breeder female, 1 dispersingmale, amb 1 female in 3 periods of her life (territorial immature, disperser and territorial breeder)) crossed it on between 4% and 33% of 45-163 monitoring days via vehicles bridge. Moreover, 4 more highways that we monitored in areas without radio-collared wolves have not delay expansion of the increasing wolf population, suggesting that these highways are not an important barrier for wolves in our study area. In contrast, only 3 of 8 wolves radio-collared < 5 km from the RDA were detected crossing it, and 2 of those 3 started to cross it only after severe habitatdisturbance; in addition, the RDA seems to have delayed wolf expansion for some 15 years, which suggests that it is a semipermeable barrier for wolves. We discuss the likely consequences of the RDA on the recovery of the Iberian wolf population. Palabras clave: Arable land, Barrier, Dispersal, Monitoring, Motorway, River, Wolf