This paper seeks to identify a mid-level unifying ethical principle that may help clarify and articulate the ethical responsibilities of business firms in the field of marketing ethics. The paper examines critically the main principles which have been proposed to date in the literature, namely consumer sovereignty, preserving the conditions of an acceptable exchange, paternalism, and the perfect competition ideal, and concludes that all of them are vulnerable to damaging criticisms. The paper articulates and defends the mutuality principle as the (...) most plausible candidate for the role of master principle in the relations between a firm and its customers. This principle requires that sellers look for reciprocity in their relations with their customers, seeking to provide to their customers something that helps them improve in some way their well-being and is commensurate with what they receive from them. (shrink)