A study was conducted in the Upper Klamath Basin, south-central Oregon, to test Luther s. Cressman' s hypothesis characterizing prehistoric Klamath Basin culture as having, from an early date, a marsh/riverine subsistence focus with long-term stability and a slow rate of internal change emphasizing intensification of the existing marsh/riverine utilization pattern. A subsistence/ settlement pattern model was developed to predict aboriginal site occurrence in the upper Klamath Basin.