System dynamics provides a powerful support mechanism for resolving problems in highly complex and dynamic contexts. Effectively building a system dynamics model in a client-group environment is a challenging task, particularly with a heterogeneous clientele and a variety of stakeholders inﬂuencing the boundaries of the model. In such environments, understanding and formulating central concerns is as important as it is challenging. This article describes a group model building initiative designed to study the implications of various policies aimed at revitalizing the ﬁshing industry in Gloucester, Massachusetts. While the iterative and interactive methods in this group modeling intervention helped ﬁshermen and community members to communicate effectively with one another, deﬁne problems and improve their understanding of the critical interactions in the system, the resulting model was used to help the Gloucester Community Development Cooperation to communicate potential impacts of a prospective ﬁsh factory to a wider audience. Copyright © 2004 John Wiley & Sons, Ltd.