For more than 10,000 years, North American coastal peoples traded with groups from the interior. Archaeological excavations provide a material record of these interactions, generally in the form of marine shell objects found at inland sites and obsidian tools at coastal sites. Numerous trade route proposals have been formulated for the procurement and movement of these goods. To test these propositions, geochemical analysis has been used for decades to source obsidian artifacts, but the same is not true for shell artifacts. Shells of the small gastropod Olivella biplicata are among the most common marine mollusc species found at interior sites in western North America. This species lives in sandy beach habitats along the Pacific coast from Alaska to Mexico and their shell was highly valued for the manufacture of beads and other ornaments throughout prehistory. Determining the geographic origin of these beads has been problematic given the large distribution of this species. However, recent research demonstrates that stable oxygen and carbon isotope analysis of shell carbonate, measured via mass spectrometry, is an effective method for determining the general geographic origin of collection for Olivella shell beads. In shell carbonate, the ratio of ¹⁸O to ¹⁶O is sensitive to salinity and temperature change, and the ratio of ¹³C to ¹²C reflects overall marine productivity. These ratios are used here to distinguish between archaeological beads manufactured from O. biplicata shells collected from southern California (south of Point Conception) and those from Northern California and Oregon. Isotopic analyses of 23 O. biplicata beads collected from five Northern Great Basin archaeological sites predominantly suggest Southern California as a primary shell source. Based on these data, I evaluate and refine existing models of prehistoric trade and interaction within the region.