We use stable isotope analysis to help determine the source locality for Olivella shells used to produce beads recovered from the Windmiller component at CC)-548. We compare isotopic profiles of modern shells collected along the coast to ancient ones. In the process, we discoverd that the Windmiller-period beads, indeed many Northern California bead types, have an unusual istotopic composition inconsistent with open Pacific coast localities, and quite unlike shells collected in southern California. The remainder of the paper examines possible source zones for Windmiller-period beads.