The external morphology and development of the immature stages of Brachyufens osborni (Dozier) (Hymenoptera: Trichogrammatidae), a Nearctic solitary endoparasitoid of the eggs of Pachnaeus spp. (Coleoptera: Curculionidae), are reported. Under our laboratory conditions (25°C and a photoperiod of 12:12 [L:D] h), B. osborni reared in eggs of Pachnaeus litus (Germar) (Entiminae) had a developmental time of 14.7 d. Egg hatching occurred within 15 h from oviposition and mandible measurements indicated the existence of two instars. First instars were mymariform, whereas second instars were sacciform. Pupation occurred 5–6 d after egg hatching and adults emerged 8 d later. On completion of the preimaginal development, the meconium was expelled and emergence took place by biting a hole in the chorion of the host egg. Both sexes emerged at the same time, and the sex ratio was female biased (3:1).