Samoa Conservation Society president James Atherton told Radio Australia the Samoa swallowtail butterfly, or Pepe Ae, was last seen there in 1979.
Mr Atherton says the butterfly can still be found in American Samoa.
He says Samoa has 30 butterfly species but the Pepe Ae was the biggest at 10 centimetres across - a blackish butterfly with white spots and also red on its wings.
