POLICE are warning shopkeepers in the Liskeard area to be aware of a fraud known as "ringing the changes'' after stores in the town were hit twice.
Police have released CCTV images of two men who conned staff in Liskeard shops last week.
The pair also defrauded businesses in Bodmin and Par on Monday and Tuesday of last week.
Last month, a staff member at a convenience store in Liskeard was confronted by two men who attempted to pull the same scam.
It involves an individual buying an item of low value, tendering a £20 or £50 note and then confusing the cashier in suggesting that they have been short-changed.
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 and he sometimes wears a tweed flat cap.
The second suspect is described as being a white man aged in his sixties. Both men are well spoken, with London accents.
A spokesman 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, please ask another member of staff to assist and double check the exchange."
Inspector Dan Ivey is warning shopkeepers to be on their "guard" and report anything suspicious as quickly as possible.
"I would advise all shopkeepers to be on their guard after police have reported several short changing scams," said the inspector, who is responsible for policing South East Cornwall.
"The police are keen to identify what they believe may be a gang travelling around the region targeting rural shops.
"We encourage shopkeepers to ring 999 so officers can attend and investigate."
If anyone has any information as to the identity of either of the men captured on CCTV, they are 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