The Arnavon Islands Marine Conservation Area (AMCA) was declared in August 1995 as a refuge for marine invertebrates. Protection from harvesting was done with agreement from, and active participation by, local communities. Protection was also facilitated by appointment of a local management committee and conservation officers. Using a sampling design based on the assessment of environmental impacts, variation in abundance of species at the AMCA and at three fished “reference” areas was established before declaration, and monitored over the next four years. The monitoring program designed to test the effectiveness of the AMCA was based on assumptions related to the effectiveness of compliance within the AMCA, patterns of exploitation outside the AMCA, the period of time required for any recovery in stocks and the suitability of habitat within the AMCA to support any such recovery. Marine invertebrates responded in different ways to the harvesting closure. Some species showed no change after 3 years, but several others showed responses that were consistent with an effect due to the AMCA. One species, Tridacna maxima, increased in abundance across the region which, without the data from reference areas, would be incorrectly interpreted as an effect due to the AMCA. The study provides valuable lessons for the management of marine reserves: It highlights the need for close collaboration with participating communities, for understanding changes in fishing practices and effort within the region and for the design of rigorous monitoring programs to test the effectiveness of marine reserves.