Police are appealing for information after a man was spotted exposing himself in woodland near Linacre Reservoirs.
A woman was walking her dog when she saw a naked man standing nearby, committing an explicit act.
The incident happened on the afternoon of September 9 in the woods, off Woodnook Lane.
The man was white with tanned skin, in his 50s, with dark grey hair and a beard. He was about 5ft 10ins in height, of slight build, and had a tattoo on his left upper arm.
Previous incidents have been reported and the man in those has been of a similar description.
Police are urging walkers who use the woods to be vigilant and to call if they see someone matching that description, or engaging in a similar act.
Anyone with information should call Derbyshire police on 101, quoting reference 16000272297.
You can also call Crimestoppers, anonymously, on 0800 555 111.