The nesting behavior of Megachile prosopidis Cockerell, Megachile chilopsidis Cockerell, and Megachile discorhina Cockerell was studied in trap nests placed near Tucson, Arizona. Quantitative differences among the five distinct nest plugs made by M. prosopidis are described. The nest plugs fall into three structural types. The first layer of capping material deposited in the first structural type is sand and an unidentified oral secretion mixed to form a hard layer. In the second structural type, the first layer is made of soft resin mixed with sand or organic matter. A third, less common, structural type includes those nests capped with pure resin. The dominant pollen host plant species was identified as Cercidium microphyllum (Torr.) Rose and Johnston (Fabaceae). Two other legumes, Prosopis velutina Wooton and Olneya tesota A. Gray, became important pollen sources late in the nesting season. Resin samples taken from the nest caps were indistinguishable, by gas chromatography, from resin of Encelia farinosa A. Gray (Asteraceae). The following parasites and predators were reared from the nests: Tricrania stansburyi Haldeman (Coleoptera: Meloidae), Lecontella gnara Wolcott (Coleoptera: Cleridae), Anthrax cintalpa Cole (Diptera: Bombyliidae), Stelis (Dolichostelis) perpulchra Crawford (Hymenoptera: Megachilidae), and Sapyga sp. Latreille (Hymenoptera: Sapygidae). New host records are reported for T. stansburyi, L. gnara, and A. cintalpa.