We present evidence indicating that the boa (Boa constrictor) was introduced onto Cozumel Island, Quintana Roo, Mexico, in 1971. This snake is now firmly established and has a wide distribution on Cozumel. We recorded an encounter rate of 1.8 boas per 100 km of forest surveyed. The introduction of the boa onto Cozumel, where it has few predators, is a threat to the existence of endemic and other native terrestrial vertebrates of the island. We recommend the following immediate actions: (1) undertake research to confirm the origin of the boa on Cozumel; (2) assess the effect of the boa on the biota of Cozumel; and, (3) if an anthropogenic origin is confirmed, an eradication programme for the boa on Cozumel should be undertaken, obtaining information on the biology and ecology of the species. Options for the destiny of the eradicated boas are discussed Español