From April 2009 to November 2010, a social marketing campaign was designed and implemented in southwest Madagascar to encourage fishers to give up destructive fishing methods and to improve the awareness and enforcement of local laws (dina). The campaign, which targeted local leaders and fishers, was designed using results from formal and informal social surveys and focused on removing locally perceived barriers to behaviour change. In this paper, we describe the campaign from design to implementation, and evaluate its effects through surveys of 500 fishers and local leaders, and preliminary observational data on dina enforcement and use of destructive fishing techniques. Results after one year showed improved knowledge and positive attitudes about dina among leaders and fishers, moderate increases in the enforcement of dina, and moderate decreases in the use of destructive fishing methods. Our findings demonstrate the power and suitability of social marketing as a tool for fostering sustainable behaviour in traditional fishing communities, when combined with good governance and enforcement strategies.