The article reconstructs the circle of persons treated as family for the purposes of the Roman inheritance tax known as the vicesima hereditatum. The regulations governing this tax did not provide a general definition of family, but the scope of tax exemptions makes it possible to identify the relationships regarded as sufficiently close to exclude a mortis causa acquisition from taxation. The principal sources are Cassius Dio and Pliny the Younger, especially chapters 37–39 of the Panegyricus Traiani. The surviving evidence indicates that the original exemption covered the closest family circle, although it remains disputed to what extent this group should be identified with the civil-law category of sui heredes. The reforms of Nerva and Trajan increased the significance of cognatic kinship and reduced the relevance of certain formal requirements, particularly patria potestas. The cognationis iura were also important, since natural kinship did not in every case suffice for the application of the exemption. The privileged group consisted primarily of parents and children, grandparents and grandchildren, and siblings, and resembled the catalogue of the decem personae without being identical to it. The analysis shows that the rules governing the vicesima hereditatum delineated a specific and functional family circle for fiscal purposes, shaped both by natural kinship and by its legal recognition.