The lagoon fishery of Rodrigues has considerable socio‐economic importance; however, catches have declined by 50% in recent years. The bluespine unicornfish, Naso unicornis (Forsskål 1775) is an important component of the fishery. To begin to assess whether marine reserves might benefit this species, sonic telemetry was used to track individual fish in one of Rodrigues' four marine reserves. Seven unicornfish were caught in the Grand Bassin reserve and tagged with abdominally implanted acoustic tags. Over a period of 57 days their locations were determined up to 21 times using a hand held hydrophone and receiver. Individual minimum convex polygon (MCP) home ranges varied from ∼10 000 m2 to ∼274 000 m2 and were not correlated with fish size. Kernel estimates of core areas (50% utilization distribution) varied from ∼5000 m2 to ∼175 000 m2. All seven fish stayed within the marine reserve and the largest home range occupied less than ∼2% of the area of the marine reserve. Fish remained on the outside edge of the lagoon on the shallow reef slope and among coral patches. These findings suggest that bluespined unicornfish biomass and numbers should increase if the Grand Bassin marine reserve is closed to fishing. Long‐term monitoring will, however, be required to demonstrate this outcome. Copyright © 2010 John Wiley & Sons, Ltd.