Copidosoma sosares (Walker) (Hymenoptera: Encyrtidae) is a specialized polyembryonic parasitic wasp native to Europe. Here we report the distribution of C. sosares from western North America. We located C. sosares parasitizing Depressaria pastinacella (Duponchel) (Lepidoptera: Elachistidae) from several sites throughout Colorado, Idaho, Montana, New Mexico, Oregon, South Dakota, Utah, and Wyoming. Specimens collected from the Netherlands and several populations in the western US were indistinguishable morphologically. Reciprocal crosses between individuals from the Netherlands and from the western US strongly suggest that US populations of this wasp are C. sosares. In view of the absence of any recorded effort to introduce this species for biological control or any other purpose, it is likely that this species was accidentally introduced along with its host sometime in the recent past.