The taxonomic status and delimitation of the genus Closterocerus Westwood is discussed, partly on the basis of new characters. The following genera are synonymized with Closterocerus: Achrysocharis Girault (maintained as subgenus), Achrysocharella Girault, Cecidiophaga Erdös, Chrysocharidia Erdös, Halochariessa Erdös, Halocharis Erdös, Moserina Delucchi, Pseudochrysocharis Girault, Wolffiella Krausse. The genus Chrysonotomyia Ashmead is regarded as a valid genus, but most species previously recognized in Chrysonotomyia are transferred to Closterocerus. The Nearctic species of Closterocerus are revised, and a key to the species is given. Twenty-one species are recognized, of which 8 are described as new: brachyphagus, carolinae, cyaneus, longus, nitidus, nius, texanus, violaceus. Four new synonyms are suggested: Chrysonotomyia baliosa Yoshimoto and C. cunicularia Yoshimoto = Closterocerus cinctipennis Ashmead; Pleurotropis tricinctus Ashmead and Closterocerus winnemanae Crawford = Closterocerus trifasciatus Westwood. Seven Nearctic species are newly transferred to Closterocerus: Achrysocharis partifuscipennis Ashmead and Moserina maculata Delucchi from Omphale, Achrysocharis phenacaspia Yoshimoto, A. spiniformis Moser, A. vesiculis Moser, Chrysonotomyia solidaginis Yoshimoto and Wolffiella ruforum Krausse all from Chrysonotomyia. The species Closterocerus damastes Walker is reassigned to Closterocerus from Chrysonotomyia. The Palearctic species Achrysocharis lyonetiae Ferrière, Nesomyia cinctiventris Ashmead, Halochariessa germanica Erdös, Entedon lanassa Walker, Achrysocharis smaragdula Graham, Chrysonotomyia yamagishii Kamijo, Halochariessa pannonica Erdös and Halocharis transsylvanica Erdös are newly transferred to Closterocerus from Chrysonotomyia. A neotype is designated for Closterocerus cinctipennis. Lectotypes are designated for Closterocerus utahensis Crawford, C. winnemanae and Pleurotropis tricinctus.