Synthetic biology represents a recent attempt to bring engineering principles and practices to working with biology. In practice, the nature of the relationship between engineering and biology in synthetic biology is a subject of ongoing debate. The disciplines of biology and engineering are typically seen to involve different ways of knowing and doing, and to embody different assumptions and objectives. Tensions between these approaches are playing out as the field of synthetic biology is being established. Here, we study negotiations between engineering and biology through the International Genetically Engineered Machine (iGEM) competition. This undergraduate competition has been important in launching and bootstrapping the field of synthetic biology, and serves as a test-bed for the engineering approach. We show how a number of issues that iGEM teams must grapple with – including standardization, design, intellectual property, and the imagination of the social – involve the negotiation of engineering, biology, and other disciplines (including computer science), in ways more complex than the engineering rhetoric of synthetic biology implies. We suggest that a new moral economy for synthetic biology is being created, in which epistemic and institutional values, conventions, and practices are being negotiated and (re)defined.