We present information on the nest-searching and parasitic behavior of the European cleptoparasitic bee Biastes emarginatus (Schenck), found attacking nests of Rophites quinquespinosus Spinola in the Czech Republic. Its mature oocyte, first instar, and last larval instar are described and illustrated by SEM micrographs, microphotographs, and diagrams. These stages are compared with those of other members of the Biastini. Because the first instar of the related Neopasites cressoni had not been described before, its description is appended, so that comparisons can be made with B. emarginatus. In most respects, the biology and immature stages of B. emarginatus closely resemble what is known concerning other tribal members, but we note that the mature larva, though agreeing morphologically with those of close relatives, has an anatomy that invites investigation into adaptive function.