A flasher indecently exposed himself to three young boys at Birchwood Nature Park in Lincoln.
The man, aged about 40 with grey and blonde spiky hair, was reported to have flashed two boys aged 11 and a 10-year-old at around 6.30pm on Wednesday, July 29.
Police are appealing for witnesses and information in tracing the man, who is described as white, between 5ft 5in to 5ft 8in height and of medium build.
He was wearing a top with vertical blue and white stripes, dark jeans and had a medium-sized black and white dog on a dark coloured lead with an attached collar.
Anyone who saw a man matching this description or witnessed anything in relation to the incident should contact Lincolnshire Police on 101 quoting incident 418 of July 29.
Alternatively, people can contact the independent charity Crimestoppers anonymously on 0800 555 111.