Fake 100 baht notes have been found at a market in Lampang province.
More than 200 vendors at Kao Chao Market were told to keep an eye out for counterfeit notes.
The notes have the serial number 0E 4647409 printed on them.
A florist at the market told police she was not aware of the forgery until she spotted another note with the same serial number. The customer who had handed over the fake was long gone by then.
According to Anchalee Luangbutr, who received the note, they are are thinner with unclear watermarks.
Police are reviewing CCTV to identify people they wish to speak to.