Filatima loowita , n. sp. is described from the Pumice Plain on Mount St. Helens in Washington, United States. An apparent specialist on lupine, early instars feed as leaf-miners, while later instars tie leaflets together into silken tunnels and feed externally, mainly on the leaf tissue. The larvae make webbed tunnels, which extend from the root bundles to the uppermost leaves of the plant. Illustrations are provided of the adult male and female genitalia, and chaetotaxal maps of the last-instar larva, supplemented with scanning electron micrographs and images of the host on the volcano.