This article examines how a group of public health physicians in the urban Amazon values medicinal plant knowledge. As biomedical health care providers, physicians routinely draw on scientific plant knowledge. At the same time, as residents of the Amazon and health care providers to the poor, they are aware of and sometimes participate in local systems of plant knowledge. When discussing medicinal plant use, physicians repeatedly mention three themes: science, superstition, and biopiracy. The way in which physicians construct and negotiate these themes is part of the process of maintaining and legitimating their expertise and authority. This analysis finds that context is key to understanding whether, when, and why physicians value certain bodies of knowledge. Locally, in clinics, scientific plant knowledge is constructed as superior. In a global context, however, local plant knowledge is explicitly valued. This situational valuation/devaluation of plant knowledge relates to the positions of power physicians occupy in each context.