Records of Diptera and their hymenopteran parasitoids are reported from a ten-year study of herbivorous insects of Nantucket Island, Massachusetts, USA, with a focus on searching for galls, leaf mines, and other characteristic feeding evidence on host plants. Our field observations and reared specimens are supplemented with herbarium records and Johnson's (1930) list of the insect fauna of Nantucket. Compared with Johnson's list of nine species of Agromyzidae, we identified 53 along with 15 others determined only to genus or higher taxa (a few of the latter could conceivably be conspecific with the former or with one another). We found 37 named species of Cecidomyiidae (including eight of the nine on Johnson's list), as well as 26 others that are undescribed or are currently unidentifiable for other reasons (a few of which could conceivably be conspecific with one of the identified species, or represent galls not actually caused by midges). A few rearings and collections of Anthomyiidae, Chloropidae, Ephydridae, Phoridae, Sphaeroceridae, and Tephritidae are reported, adding another seven species and two genera to Johnson's list. Cerodontha (Dizygomyza) edithae Eiseman and Lonsdale (Agromyzidae), an as yet undescribed species of Ophiomyia Braschnikov (Agromyzidae), Megaselia nantucketensis Eiseman and Hartop (Phoridae), and the cecidomyiid parasitoids Platygaster tephrosiae Buhl and Eiseman and P. vitisiellae Buhl and Eiseman (Hymenoptera: Platygastridae) are known only from specimens reared as part of this study, which also produced paratypes of Liriomyza pistilla Lonsdale (Agromyzidae). Noteworthy cecidomyiid records include the first known specimens of an undescribed Asphondylia Loew on Solidago sempervirens L. (Asteraceae), galls of an undetermined lasiopterid species on Tephrosia virginiana (L.) Pers. (Fabaceae) that have only been found on Nantucket, and previously unreported galls on Gaylussacia baccata (Wangenh.) K. Koch (Ericaceae), Ionactis linariifolia (L.) Greene (Asteraceae) (Dasineura Rondani sp.), Quercus prinoides Willd. (Fagaceae), Salix purpurea L. (Salicaceae), and Solidago latissimifolia Mill. (Asteraceae) (Asphondylia sp., Rhopalomyia Rübsaamen sp.).