The ground plan structure of the labium of the Apocrita and Chalcidoidea was characterized by reference to the structure of the labrum in Symphyta. The ground plan configuration in Chalcidoidea is regarded as flaplike, with many evenly distributed setae and a lobate epipharynx. Within Chalcidoidea, the structure of the labrum was compared in exemplars of Eucharitidae, Perilampidae, and most of the subfamilies currently referred to the Pteromalidae. The digitate labrum of Perilampidae and Eucharitidae is regarded as the first synapomorphy of the two families from the adult stage. The digits of the labrum intermesh with setae of the labiomaxillary complex to form a sievelike apparatus that may exclude pollen and are hypothesized as an adaptation for nectar feeding. The digitate labrum and the flaplike labrum with marginal setae of Chrysolampinae share derived characters: stout setae, reinforced setal bases, and distinct sockets. A transformation series is proposed that derives the digitate labrum from the chrysolampine labrum by the extension of processes from the margin. A survey of the structure of the labrum in Pteromalidae failed to refute this hypothesis of synapomorphy; the labrum of Eutrichosomatinae is similar in many regards but is regarded as independently derived. Three distinct configurations of the labrum are shared by subfamilies and tribes of Pteromalidae: (i) Cleonyminae, Chalcedectinae, Brachyscelidiphaginae, Cerocephalinae, and Macromesinae; (ii) Spalangiinae, Asaphinae, Ceinae, and Miscogasterinae (Ormocerini); and (iii) Miscogasterinae (Sphegigasterini, Miscogasterini) and Pteromalinae. The following subfamilies have unique configurations of the labrum: Chryoslampinae, Diparinae, Eutrichosomatinae, Cratominae, Colotrechninae, and Eunotinae. Hypotheses of transformation of the labrum were suggested for the following: Eutrichosomatinae to Miscogasterinae (Sphegigasterini, Miscogasterini), Pteromalinae; Diparinae to Miscogasterinae (Sphegigasterini, Miscogasterini), Pteromalinae; and Diparinae to Cratominae. The analysis makes explicit use of a priori rules of character transformation as a basis for testable hypotheses of relationships. This method is considered the most appropriate for evaluating complex morphological data sets and should allow progress toward a natural classification in groups, such as the Chalcidoidea, that do not have an explicit phylogenetic hypothesis as a point of departure.