Technical Abstract: Locoweeds are defined as Astragalus and Oxytropis species that cause intoxication from the alkaloid swainsonine. Swainsonine concentrations in Oxytropis sericea are influenced by location, plant part, and the developmental stage of the plant. Swainsonine concentrations follow similar trends at each location generally increasing over the growing season in above ground parts until the plant reaches maturity and staying the same in the crowns. As the plant begins to senesce, swainsonine concentrations decrease in the floral parts by greater than half of their peak concentration. Swainsonine concentrations in Oxytropis sericea are positively associated with the endophyte Unidfilum oxytropis, which has been shown to be responsible for swainsonine biosynthesis. Similar to swainsonine concentrations, endophyte amounts are influenced by location, plant part, and the developmental stage of the plant. Likewise, endophyte amounts generally increase over the growing season in above ground parts and remain static in the crowns at all four locations.