Manuel Segovia, 75, and Isidro Velazquez, 69, are the only two people who can speak it fluently, but they will not talk to each other, despite living only 500 yards from each other in the village of Ayapa in the Mexican southern state of Tabasco.
Mr Segovia reportedly spoke the language with his brother until he died around 10 years ago, and he still converses with his son and wife, who understand him but are unable to speak more than a few words. Mr Velazquez is understood to not speak to anyone in the language. The men are also said to disagree over aspects of the language.
It is not known why the pair do not talk to each other, but a linguistic anthropologist from Indiana University who is involved in a project to make the first dictionary of the language, has said they do not have a lot in common.