Astragalus (Fabaceae) is a broadly distributed, diverse, and economically important group of plants. Given the number of species and its distribution, it is not surprising that there are many species that are highly restricted and endangered. Among these is the Ash Meadows milkvetch, Astragalus phoenix. Here we investigate the breeding biology of As. phoenix. Our data show that As. phoenix is xenogamous and that Anthophora porterae (Hymenoptera: Apidae) is the most likely pollinator. We also noticed Apis mellifera visiting As. phoenix, though it appears unlikely that Ap. rnellifera contributes significantly to the reproductive success of As. phoenix. We located a nesting aggregation of An. porterae and offer a description of its nest architecture.