The genital segments and internal genitalia of Agathiphaga vitiensis are described. Sternum VIII is anteriorly produced into blunt paired apophyses and posteriorly into a tongue-shaped lobe. Segment IX is a complete ring, very short in the dorsal and ventral midlines; its anterolateral lobes are largely apodemal. The long and curved gonopod ("valva") consists of a single piece. There is no median sclerite between the gonopod bases, but an open, softwalled "subgenital crypt" below the entrance of the phallocrypt may be homologous with the "median plate" in other primitive homoneurous moths. Tergum X bears a pair of broad "superior lobes" and the postgenital complex terminates in a medially intended, sclerotized "terminal lobe" above the eversible perianal area. The roof of the posterior part of the genital chamber bears a median aggregation of cuticular spines (the "spiny plate"), and a pair of smooth lateral sclerotizations ("presocii") tentatively attributed to venter X: a pair of setose sclerites (socii) are tentatively attributed to the paraprocts. The area bearing the spiny plate and presocii may in repose be folded down behind the phallus, thereby closing the phallocrypt. The phallus comprises a tubular phallotheca and an eversible aedeagus; the thick basal margen of the phallotheca is posteriorly expanded and forms the floor of the greater part of the phallocrypt; there is no ventral aedeagal branch. The musculature comprises two IX/X muscles, a segment IX muscle inserting on the subgenital crypt, phallic pro- and retractors (the former originating in the gonopod), intrinsic phallic muscles, a single segment IX muscle (adductor) to the gonopod and five intrinsic muscles of the postgenital complex. Each testis comprises four large, separate follicles. The spermatozoa do not remain grouped in discrete bundles in the vas deferens. Seminal vesicles are located on the vasa deferentia close to the testis and are doubtfully homologous with the vesicles in other Lepidoptera. The unpaired ejaculatory duct is very short. The evidence bearing on a reconstruction of the ground plan of the lepidopteran male genitalia is reviewed. Segment VIII was similar to the preceding segments. It is tentatively suggested that tergum and sternum IX were fused, that the gonopod was undivided and that a tubular, partly sclerotized aedeagus was present; deviations from these states within the order are therefore considered to be autapomorphic. The base of the aedeagus was probably surrounded by a short, collarlike phallotheca. It is suggested that there was a median sclerite between the gonopod bases, but the presence of discrete, paired and muscular "valvellae" in the lepidopteran ground plan is considered doubtful. It is further suggested that dorsum X bore a pair of lobes and that there were paired sclerotizations in venter X. The X/XI boundary is very difficult to trace. Seventeen muscle sets are ascribed to the lepidopteran ground plan; it is considered an autapomorphy of this ground plan that the phallic protractor originates within the gonopod. The testes presumably had large, separate follicles and there may have been two pairs of tubular accessory glands. The testes and the double set of accessory glands of Agathiphaga could be cited in support of a sistergroup relationship to all other Lepidoptera whereas the phallic structure (and possibly the "spiny plate") might support a sister group relationship to the Heterobathmiina. There is no support in male genital structure for a sistergroup relationship to the Heterobathmiina + Glossata; the latter phylogenetic hypothesis may be preferable on other grounds, however.