Two South African black rhinos were found dead in mid-October in Zakouma Park in south-eastern Chad. The rhinos were reintroduced in the park alongside 4 others in May.
African Parks NGO said on Sunday, that the male and female rhinos were not poached, though the exact cause of their death is not yet known.
The four others still alive are under close surveillance.
A joint initiative by the Governments of Chad and South Africa resulted in the reintroduction of six black rhinos into Zakouma Park in May.
This reintroduction was to initiate a new outbreak of black rhinos, which were last seen in the area in 1972.
Thousands of rhinos are slaughtered in Africa annually for their horns, which are highly prized by traditional Chinese medical practitioners.
There are still 5000 specimens of black rhinos on the African continent, including nearly 1900 in South Africa.