In 1998 and 1999, relict populations of the Nile crocodile Crocodylus niloticus were rediscovered in south-eastern Mauritania, 70 years after the last reports of their existence and 6 years after the IUCN Species Survival Commission listed them as extirpated in that country. Crocodiles' were found in four different types of wetlands, herein described. Preliminary observations on their natural history and their interactions with humans are provided, and the main threats and the prerequisites for their protection are discussed.