In this paper, we develop an approach to the study of whistleblowing as a critical practice that is involved in the contestation of truth and power in the workplace. We situate our analysis in the context of practice-based thinking and specify the social practice of whistleblowing with reference to Michel Foucault’s concept of ‘parrhesia’ (frank speech). We then introduce the case of Guido Strack, a former European Union official who worked as section leader at the Office des Publications Officielles des Communautés Européenne from 1995 to 2002. Strack spoke out against malpractice in the EU in 2001 and officially reported alleged financial misconduct in 2002. In our analysis, we focus on the interplay between and effects of different modes of truth-telling in the context of this specific organization – a context marked by the uneasy coexistence of different normative and discursive frames. We argue that the parrhesiastic modality of truth-telling threatens the established ‘working solutions’ that reconcile the tensions inherent in the regime of practices and thus introduces a ‘critical opening’ that harbours the potential for both personal and organizational transformation. We conclude by highlighting the potential of a nuanced understanding of parrhesia for studying ‘critical practices’ more generally.