Astragalus diaphanus is a rare plant endemic to the John Day River drainage of north-central Oregon. This species has several interesting features, including the dimorphism of its fruit and its geographical isolation from the two nearest taxonomically related species, which occur in Colorado. This study addressed the species' distribution and habitat, the taxonomic relationships between the varieties of A. diaphanus, certain morphological comparisons among the species, possible reasons for the rarity of A. diaphanus, and the population biology of this taxon. Astragalus diaphanus was found to be more widespread in the John Day drainage than was previously known, but its range has shrunk due to habitat loss along the Columbia River. In this study, two varieties are recognized within a single species, based on striking morphological differences in pod forms which correspond to a break in geographical distribution. Other morphological characters are similar between the varieties. Flavonoid analysis and chromosome counts support this taxonomic treatment. Further study is needed to elucidate the relationships of A. diaphanus and its taxonomic relatives in Colorado. A low reproductive rate in A. diaphanus appears to be a potential problem, possibly contributing to its rarity. The species exhibits a combination of annual and biennial lifecycles. Many annual individuals of A. diaphanus perish without reproducing. This may be off-set by a large seedbank, which is replenished sporadically by high production in robust biennials. There is not a need at the present time to provide legal protection to var. diaphanus, the more widespread and common taxon. However, var. diurnus has a very limited distribution along the South Fork of the John Day River and is represented by few populations. Many of these are marginally viable, and there are active threats to the existence of the latter variety. Astragalus diaphanus var. diurnus should be afforded legal protection as an endangered species. / Graduation date: 1991