This article studies a conflict over two competing norms in which the actors demonstrated incompatible positions not through arguments, but through actions. During the SARS crisis, China and the World Health Organization (WHO) entered a norm conflict over the precedence of sovereignty or global health security. Both resorted to behavioural, not discursive contestation: while the WHO practically but not rhetorically challenged the sovereignty norm by acting according to the norm of global health security, China—without openly acknowledging it—contravened the basic principles of global health security by acting according to the overlapping sovereignty norm. Why and with what consequences do actors choose to contest norms through actions rather than words? The article accounts for the resort to behavioural contestation by pointing to the strategic advantages it offers for furthering a contentious norm understanding without facing the social costs of making it explicit. It furthermore highlights that behavioural contestation may feed back into and change the odds of discursive contestation as its practical effects provide rhetorical resources to (de-)legitimate one or the other position. The propositions are illustrated in the interactions of China and the WHO during the SARS crisis and the subsequent norm development. This article forms part of the special section of the May 2019 issue of International Affairs on ‘The dynamics of dissent’, guest-edited by Anette Stimmer and Lea Wisken.