The Siona and Secoya Indians of eastern Ecuador are shifting cultivators and foragers who make extensive use of both feral and domesticated plant materials in most aspects of their culture. This study fully or partially identifies approximately 224 species in 1 66 genera and 69 families that are in common occurrence. A wide variety of these plants are employed for foods and as materials for tools, weapons, crafts, construction, and personal adornment. The use of plants for medicinal and ritual purposes is notable, with emphasis given to hallucinogenic plants of such genera as Banisteriopsis, Diplopterys, Brugmansia, and Brunfelsia. As in a number of northwestern Amazonian societies, the use of Banisteriopsis is particularly significant because it is the basis of the most important rituals and is viewed as the medium through which supernatural knowledge and power are achieved. The Banisteriopsis cult is the conceptual cornerstone of Siona and Secoya religion, mythology, art, medicine, and warfare.