Wildlife experts fear that the adder may be extinct in Nottinghamshire after another annual survey failed to find any evidence of the reptile.
The last confirmed sighting of the venomous snake in the county was in Sherwood Forest in 2005.
Nick Crouch, a conservationist, said its disappearance was "very worrying" as the species was a key indicator of the health of the food chain.
The wildlife expect has appealed for the public to report any sightings.
'Bit of a mystery'
Mr Crouch, a nature conservation leader for Nottinghamshire County Council, said the species was once quite widespread across Sherwood.
Adders are in rapid decline in the UK and experts put this mainly down to habitat loss and disturbance.
"These snakes require areas of heathland and woodland," Mr Crouch said.
"These areas are being lost or are not being managed properly but quite why the adder seems to have disappeared in Nottinghamshire is a bit of a mystery.
"There's still plenty of suitable habitat left in the county including where the snake was recorded in 2005."
Nottinghamshire County Council is encouraging people to report all sightings of reptiles in the county, including grass snake, slow worm and common lizard.
"Such information helps us to identify important areas for these species, and to target action to conserve them," Mr Crouch said.
All sightings should be reported to the Nottinghamshire Biological and Geological Records Centre at Wollaton Hall.