Herein we provide the first account of the nesting biology of Osmia (Helicosmia) chalybea Smith and of its cleptoparasite Stelis (Stelis) ater Mitchell, a newly confirmed host association. The nesting behavior of O. chalybea is similar to what is known about other members of the subgenus Helicosmia, but novel information concerning egg eclosion and cocoon structure and function are reported. Eggs of S. ater were discovered both on the front surface of the host provisions as well as deeply buried in the pollen-nectar mixture. From numerous observations, larval S. ater attacks host immatures or conspecifics whenever encountered. Because young larvae are unable to crawl (perhaps a family characteristic), the hospicidal individual may be in its third to fifth stadium or even in its second stadium, although none of the latter was certainly identified. Although usually a single cleptoparasitic egg or larva was found in a cell, two or three were occasionally observed there. The cocoon of S. ater was also examined closely. Although its construction differs in a number of ways from that of the host cocoon, the functions of both seem to be the same, i.e., excluding parasitic/parasitoid arthropods and preventing desiccation while allowing exchange of air with the outside through complex filtering devices at the front ends of cocoons. Known parasites/parasitoids in the area of study include the minute parasitoid wasp Melittobia digitata Dahms (Eulophidae) and the parasitic mite Chaetodactylus rozeni Klimov and O'Connor (the latter recovered from adults of O. chalybea). The nearly airtight cocoon fabric of both bee cocoons presumably assists immatures in maintaining their water balance through their many months of confinement.Ovarian statistics for both species are presented and compared with congeners, and their eggs/mature oocytes are described. Both have five larval instars. Comparative taxonomic descriptions of last larval instars are presented, and mandibular shapes of other larval instars are diagrammed.