Biologies of eight chrysidid species were described. These wasps parasitized bees and wasps collected in trap nests set at several sites in two canyons near Logan, Utah. The completed nests were taken to the laboratory and the parasitized ones were kept for study of the development a l biology and behavior of both host and parasites. The larval stages of Chrysura smaragdicolor, Chrysura sonorensis, Chrysis parkeri, Chrysis derivata, Chrysis coerulans, Cerotachrysis enhuycki, Trichrysis doriae and Hedychridium solierellae were studied. Omalus iridescens and Omalus purpuratus larvae were observed only in the fifth instar. The major differences between species or genera were found in the first instar larvae. The other larval instars differed in size but behaved similarly. The first instar larvae had a sclerotized, prognathous head, sharp sickle shape mandibles. When threatened the parasite larvae could escape quickly with the help of the forked appendages of the first segment. All the species except in Chrysura killed the host in the first instar and fed on the prey stored by their hosts. The Chrysura first instar attacked the host bee larva only after the latter had spun its cocoon. By the fifth instar all species of larvae had developed several teeth (two to three) at the distal end of each mandible. The hardening of these teeth coincided with the change of the food consistency from liquid to solid. The study also included a proposed evolutionary scheme of the possible pathway of morphological and behavioral characters that might ensure more successful parasitism.