In this essay, I examine the controversy concerning the advocacy of ethical values in conservation biology. First, I argue, as others have, that conservation biology is a science laden with values both ethical and non-ethical. Second, after clarifying the notion of advocacy at work, I contend that conservation biologists should advocate the preservation of biological diversity. Third, I explore what ethical grounds should be used for advocating the preservation of ecological systems by conservation biologists. I argue that conservation biologists should (...) defend their preservationist positions on instrumentalist grounds alone if the context of discussion and debate is a scientific one. (shrink)