Dalmatian toadflax (Linaria dalmatica) is a noxious weed of range and fallow lands in the western U.S. The stem-mining weevil Mecinus janthinus Germar was first introduced in 1991 for the biological control of toadflax. Weevils readily establish at Dalmatian toadflax release sites. However, at present there is little data available on the impact of the weevil on its host plant. We set up a study to assess the impact of the weevil on Dalmatian toadflax populations throughout the state of Idaho and in eastern Washington. We chose only study sites at which 200 or 400 weevils were released. At each study site, we established 44 permanent 0.125 mē plots along four parallel 48 m long transects that were placed around the original release point of the biological control agent. In each plot, we measured Dalmatian toadflax stem density and cover, other plant cover, and bare ground and litter during May 2004 and 2005. All Dalmatian toadflax flowering stems from the previous year were removed from all plots and dissected for M. janthinus attack and mortality rates. First results show that weevil attack rates and weevil abundance generally increased at study sites. In northern Idaho, weevil numbers increased more than 4,000% within four years. High weevil densities cause a sharp decline of Dalmatian toadflax stem density and cover. The decline of Dalmatian toadflax plant density did not necessarily trigger an increase of forbs and/or grass cover. Open space was frequently invaded by other exotic plants.