The work lasts a week and will finish on Friday
The first-ever comprehensive survey of the rare lichens growing on Stonehenge has begun.
Internationally-renowned ecologists are on site trying to find out what species of lichen are growing on the stones, and what this tells us about the natural history of the site
The survey will enable these species to be preserved in the future.
Ecologists hope the number of known lichen species will have increased - at the last count there were 79 species on the stones.
Alan Cathersides, English Heritage senior landscape manager, said: "This survey is part of a continuing process to help us understand more about the history and ecology of Stonehenge.
"Hopefully it will answer some of the questions on how these unique lichen communities have developed."
The work will continue until Friday.