Fake £20 and £50 notes are being used throughout Llanelli , Dyfed-Powys Police have said.
A spokeswoman for the force said they had received 15 reports within a week of fake notes being used in Parc Trostre and the town centre.
Police have warned shoppers and businesses to remain vigilant when handling cash, and in particular £20 notes.
A spokeswoman for Dyfed-Powys Police said: "There have been around 15 reports to police in the past week and that is only the ones that police are made aware of.
"Reports have mainly come from shops at Trostre, but the town centre is also experiencing the issue."
Businesses or shoppers with concerns about fake money being used have been urged to refuse the sale and contact the police.
A tweet from Llanelli's neighbourhood policing team read: "Notice to shoppers and stores around the Llanelli area, there have been reports of fake notes circulating Parc Trostre, ensure you check your money and be vigilant in reporting anything suspicious to us."
Anyone with information is urged to contact the police on 101 or Crimestoppers anonymously on 0800 555111.
