ABSTRACT 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 als...