The integration of environmental concerns into agricultural policies - through agri-environment measures (AEM) - has seen a fast development across Europe. This paper conceives AEM as an evolving instrument, a product that takes shape, gets diffused and taken up in, by and through networks of relations. Success then depends on the mobilisation or active participation of all those who may support and develop it. Using the examples of the Flanders' and Walloon regions of Belgium, the paper sets out to examine the mechanisms by which mobilisation for agri-environmental management develops, and by doing so, to gain a better understanding of mobilisation capacity as a concept to be used for evaluating policy implementation in this area. The study follows AEM along the various trajectories of implementation (design, distribution, application). The findings reveal how mobilisation capacity is gradually built-up by the interplay between AEM and the networks it connects to. The case illustrates well how such interactions occur all the way from administration offices to farmers' fields, and that their nature can be very different (e.g., formal and informal, durable and short-lived, expected and unexpected). It is concluded that in evaluating actor-networks, one should look at them in an open and fluid manner, that is, not to privilege any particular configuration or form of attachment over the other, not take intentions and objectives as a starting point but instead address the opportunities for synergies, and be aware that any network built around the instrument may change its content and the way it functions.