N. achilleae is a bivoltine fruit fly developing solely in the flower heads of Stephanomeria spp. belonging to the subtribe Stephanomeriinae of the tribe Lactuceae in southern California. This species is distributed along the east coast of the United States and in California and Arizona, but has not been reported from middle America. Moreover, in the East, it is reported from 17 other host plant species in the subtribes Asterinae, Hieraciinae, Lactucinae, Liatrinae, and Solidaginae of the tribes Astereae, Eupatorieae, and Lactuceae, but not from Stephanomeriinae, which are mostly western plant species. The second- and third-instar larvae and puparium are described, and these larval instars are compared with those of other Neaspilota species. The anterior thoracic spiracle of the second instar has 3 papillae, but 3 and 4 papillae are reported in the third instar. The second and third instars of N. achilleae have an undetermined number of oral ridges with dentate posterior margins in a vertical series lateral to the oral cavity. The appearance and arrangement of these oral ridges is now known to be a distinguishing generic larval character. The numbers of these oral ridges, along with the patterns of minute acanthae circumscribing the body segments, are useful in distinguishing several species of Neaspilota larvae. The larvae feed mainly on the corollas of florets as first and second instars; however, as third instars, they may extend their feeding into and through the ovules or soft achenes, and sometimes into the receptacle, and supplement their diet with sap. At least 2 annual generations are produced in southern California. The life cycle is of the aggregative type and overwintering mainly occurs as sexually immature adults. Pteromalus sp. was reared as a solitary, larval-pupal endoparasitoid of N. achilleae; Eurytoma obtusiventrus or near and E. veronica, as probable, solitary, larval-pupal endoparasitoids.