Corallus cropanii - also known as Cropan's boa - is known from only a handful of dead specimens collected after the snake was first seen and described in 1953.
The slippery serpent was captured in January - the second living specimen ever seen, and the first glimpsed in 64 years.
The boa was brought to local scientists by rural residents of the Ribeira Valley Region in Brazil.
Cropan's boa is found only in Brazil's Atlantic Forest, in a 300 square kilometers region in Sao Paulo.
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It is the rarest type of boa in the New World and possibly the rarest on Earth, according to the International Union for Conservation of Nature and Natural Resources.
The species was described in 1953 from a single adult male specimen. Cropan's boas have never been observed in the wild.
When the recently captured female is released, it will be carrying an implanted radio transmitter, which could provide scientists with a few more clues about how these snakes survive in their forest home.
"It will be released in its natural habitat and receive equipment with radiotelemetry that will enable its tracking in nature and the transmission of information to researchers," she said.