The claim that public problems are constructs is now widely recognized as justified and was first established in social problem theory. The high instability of problem definition activities in the case of water pollution coming from agriculture in Brittany demonstrates this particularly well. The objective of this article is to describe ways by which an environmental movement organization (EMO) conceives its activities of public problem construction. Inspired by social movement theory, but aiming at overcoming its weaknesses, the paper seeks to highlight the influence of EMO endogenous meaning production in problem construction processes, a dimension often overlooked even by framing theory. In a bid to support that claim, the paper shows the influence of the affective dimension over the strategic one within the problematization process. Forging a conceptual distinction characterized by a perceived problem and strategic definitions, the paper underlines the fundamental interrelated nature of these two components and consequently emphasizes the reciprocal dependence of the perceived problem over the strategic (either material or cognitive) definitions. Finally, the paper evokes the benefit and impact of this conceptual distinction on the policy-making process. (Résumé d'auteur)