Abstract.A new genus of the subfamily Entedoninae (Hymenoptera: Chalcidoidea: Eulophidae),Perditomlus, is described. Male genitalia in species of this genus display extraordinary morphological differences to an extent which is unique in the Chalcidoidea, and the utility of this character and of male antenna in species identification is demonstrated. The value of male genitalia in chalcid systematics is discussed, and a possible reason for the exceptional development inPerditomlusis considered.Perditomlusincludes thirty‐three species, unless otherwise stated from Mexico:abruptus, angustatus, avirostris, bidenticulatus, bifidus, bilobus, calcaratus, crassiscapus, evanescens, falcispinus, faretus, flexilis, geniculates, grandiculus, hastatus, insternatus, languidiscapus, longiparameratus, magnicaulis, mucronatus, parameratulus, penicillatus(Mexico, U.S.A., Canada),pilosus, pinguiscapus(U.S.A.),planiscapus, porcodon, punctiscapus, sinuiscapus, trispinus. unispinus, verticillatus, woolleyi(Mexico, U.S.A.), andzolnerowichi. Perditomlusis considered most closely related toOmphaleHalida