The Caribbean Islands are a biodiversity hotspot where anthropogenic disturbances have had a significant impact, causing population declines and extinction of endemic species. The ground snake Erythrolamprus cursor is a dipsadid endemic to Martinique; it is categorized as Critically Endangered on the IUCN Red List and is known only from museum specimens. The snake was common on Martinique during the 18th and 19th centuries but there have been no reliable sightings since 1968, suggesting it may have gone extinct, probably as a result of the introduction of the small Indian mongoose Herpestes javanicus auropunctatus. However, the islet known as Diamond Rock, south-west of Martinique, is mongoose-free and the last reported sighting of E. cursor there was in 1968. The islet was last occupied during the Napoleonic Empire (the early 19th century), is now completely protected, and is difficult to access (it spans 5.8 ha, with a maximum elevation of 175 m). We conducted the first extensive survey of the islet, over 10 days, to clarifty the status of E. cursor. Our study revealed that unique conditions exist on Diamond Rock (i.e. aridity and a distinct potential prey community) and that E. cursor would have had to modify its ecology to persist on the islet. Although the rugged terrain of Diamond Rock makes it difficult to explore, it is probable that E. cursor is now extinct.