A decade of intermittent archaeological research in the upper Zaña Valley in northern Peru has documented an intensive Middle Preceramic period (ca. 6000—4200 B.C.) occupation in the tropical-forest and thorn-forest ecotone on the western Andean slopes. This research has revealed one stratified nonresidential site (the Cementerio de Nanchoc), characterized by dual earth mounds, and a complex of small, preceramic residential sites in the Nanchoc branch of the valley. The nonresidential site is associated with the production of lime, probably used as a mineral supplement to the diet or as an extractive agent with coca leaves. Evidence recovered from residential sites shows that incipient horticulturists, documented by the presence of several species of cultigens, and unspecialized hunters and gathers lived in scattered households located along small streams in alluvial fans above the valley floor. A unifacial lithic technology and a diversified ground-stone technology attest to an economy primarily adapted to plant resources. The preceramic culture of the upper Zaña Valley is interpreted as a local manifestation of an early western-slope-forest cultural tradition associated with the development of a specialized public precinct and the adoption and intensification of agriculture.