This thesis examines phonological substrate interference as a result of language shift. It has been observed that Scandinavian-American communities in the Upper Midwest, where Norwegian, Swedish, and Danish speakers shifted to English in the 19^th and 20th centuries, devoice /z/. This phenomenon is thought to be due to the lack of a voicing contrast in sibilants in Scandinavian languages. Acoustic analysis was performed comparing the production of /s/ and /z/ in a highly Scandinavian region, the Red River Valley of North Dakota and Minnesota, and a region with very little Scandinavian presence, the Piedmont of North Carolina. Red River Valley residents with and without Scandinavian background were likewise compared. It was found in this study that the speakers with a greater degree of Scandinavian background produced less glottal pulsing in /z/ and more in /s/ than other speakers. The latter result had not been previously recorded. I therefore propose that the substrate effect is not devoicing of /z/, but greater neutralization of the voicing contrast.