This study investigates a ‘dark-side’ of supplier–buyer relationships, specifically the links between supplier development initiatives, relational norms and supplier opportunism, utilizing thematic analysis and qualitative comparative analysis. While buyers often employ supplier development initiatives to improve procurement, they can be ineffective and stimulate opportunistic behaviour by suppliers. Drawing on the case of agri-food supply chains in Vietnam, the paper analyses the relationships between specific supplier development initiatives and forms of opportunism, considering the role of relational norms. While often regarded as reducing the likelihood of opportunism, this study identifies that relational norms may include norms of opportunism in supply chain relationships, which sanction a degree of opportunistic behaviour. The study contributes to supply chain management theory and practice by investigating how buyers can address opportunism, so that supplier development initiatives curb supplier opportunism rather than trigger it.