Parasitization of larch casebearer, Coleophora laricella (Hübner), by the internal larval parasite, Agathis pumila (Ratz.), commonly has been found to decrease between successive samples in the spring. The decrease has been attributed in part to mortality of A. pumila, which prolongs the casebearer's larval stage, thereby exposing it to subsequent parasitism by other species that are intrinsically superior (Smith 1929) and to secondary parasitism of A. pumila (Graham 1949; Quednau 1970; Raske and Schooley 1979; Ismail and Long 1982). Another reason may exist for this decrease in parasitization by A. pumila: A higher percentage of parasitized than unparasitized larvae may descend from the larch foliage to the understory vegetation, which is not in the normally sampled universe (Webb 1953; Moody 1977; Denton 1979; Long and Theroux 1979; Ryan 1983). Eidmann (1965) reports that casebearers are often found on the undergrowth when high populations strip the trees, but prior to this no one had reported any connection with parasitism. This movement may result in a decreased percentage of parasitized larvae remaining on the larch foliage. Evidence presented here substantiates this hypothesis for one location, but for a more general conclusion other areas should also be examined.