The sharks are often seen off Cornwall in the summer
Volunteers are being asked to monitor the sea off Cornwall to see if they spot any basking sharks.
The survey, the third successive one by the Cornwall Wildlife Trust, will be based near Land's End and run from the beginning of June until October.
Those involved will monitor sharks from a headland overlooking the Runnel Stone on the Land's End peninsula.
In 2009, 64 volunteers took part in cliff-top surveys, recording almost 1,000 sightings, the trust said.
Basking sharks can grow up to 10m (33ft) in length and weigh up to seven tonnes.
The sea off the Runnel Stone is a favourite area for the sharks to feed. They use their giant mouths to filter seawater for plankton, and do not eat anything larger.