Aphantorhaphopsis samarensis (Villeneuve), a European tachinid, has been released in North America for classical biological control of the gypsy moth, Lymantria dispar (L.). This study examined the host range of A. samarensis. We used three approaches: (1) field collection and rearing of potential alternate or alternative hosts at European sites where A. samarensis was known to occur, (2) choice tests offering females of A. samarensis both gypsy moth and native North American species of Lepidoptera, and (3) host suitability tests in which we artificially inoculated European nontarget species with mature eggs of A. samarensis dissected from gravid females. In the field studies, we collected a total of 851 caterpillars, belonging to at least 54 species other than gypsy moth in 11 families, over several years, but none yielded A. samarensis, with the possible exception of a single larva of Lymantria monacha (L.) and the rusty tussock moth, Orgyia antiqua (L.), which yielded puparia resembling those of A. samarensis. In laboratory tests, we offered females of A. samarensis 11 native species of North American Lepidoptera in five families, but only the lymantriid Orgyia leucostigma (J. E. Smith), was successfully parasitized. In host suitability studies, we inoculated 10 species of Lepidoptera in eight families with mature eggs of A. samarensis, but parasitism was successful only in L. dispar. We conclude that A. samarensis has a high degree of host specificity.