POLICE are warning shop staff in the Bodmin area to be aware of a fraud known as "ringing the changes" after stores in the town were hit twice last week.
Police have released CCTV images of two men who conned staff in Bodmin.
The pair also defrauded businesses in Liskeard and Par on Monday and Tuesday of last week.
Last month, a staff member at a convenience store in Wadebridge was confronted by two men who attempted to pull the same scam.
The "ringing the changes" fraud involves the suspects visiting shops and making a small purchase and, while the till is open, requesting the cashier to change various notes for other denominations, and in so doing receiving more money in change than they are due.
During the transaction staff become confused, and later discover the discrepancy.
The first suspect is described as being Mediterranean in appearance, aged in his thirties, with dark hair .He sometimes wears a tweed flat cap.
The second suspect is described as being a white man in his sixties.
Both men are well-spoken with London accents.
A spokeswoman for Devon and Cornwall Police said: "We are advising shop-owners to warn their staff of the scam and that if someone asks for change in this manner, to ask another member of staff to assist and double-check the exchange."
The incident in Wadebridge occurred when two men entered the shop with a number of £20 notes and asked the staff member to change them into £10 notes.
As the cashier started to do this one of the men asked for them to be in different notes, but the transaction was aborted and the men were asked to leave the shop.
The male became very insistent that the cashier complete the money exchange and initially refused to leave the shop.
Sergeant Robin Moorcroft is warning shopkeepers to be vigilant and to report anything suspicious as quickly as possible.
"Should you see any suspicious individuals in your premises, particularly individuals attempting to 'ring the changes', please call 999 for police attendance," Mr Moorcroft said.
"It would be helpful to have a description of the individuals and if you see them getting into a vehicle, then note the registration number."
Anyone with any information as to the identities of either of the men captured on CCTV is being asked to contact police on 101 quoting police reference BD/14/280 or alternatively call Crimestoppers anonymously on 0800 555111 or report online at www.Crimestoppers-uk.org