We estimate that between 25,000 and 35,000 long-tailed macaques, Macaca fascicularis, live on the island of Mauritius, 1,865 km2, in the western Indian Ocean, and we detail their distribution on the island. Probably introduced to Mauritius at some time in the 16th century, the macaques have been implicated by a succession of authors as agents both in the extinction of the bulk of the island’s unique vertebrate fauna, and in the destruction of its indigenous vegetation. Some of them have gone so far as to suggest that in view of their depredations the macaques should be eradicated from Mauritius. However, studies of the behavior and ecology of the macaques in both degraded savanna and native forest habitats, supplemented by surveys around the island, cast doubt upon this putative destructive role. Macaque population densities in Mauritius range from a maximum of around 1.3 individuals/ha in degraded savanna formations, to a minimum of approximately 0.33 individuals/ha in indigenous forest, and reflect a clear preference among these primates for secondary environments. Thus the long-tailed macaques in Mauritius, as in their southeast Asian homeland, best fit the profile of a ‘weed’ species that has simply exploited the disturbance by humans of the indigenous vegetation. They do not at the present time appear to pose a major threat to what remains of the remarkable indigenous vertebrate fauna of Mauritius, although they may help to disperse the seeds of invading exotic plant species in what remains of the indigenous forests.