Highway police arrested a man with 66,000 methamphetamine pills in his possession in Chumphon province on Tuesday.
Pol Col Apichart Pochan, commander of the 2nd Highway Police Division, identified the man as Adam or Ma Maseng, 27, of Songkhla's Thepha district.
Police stopped a Honda Accord car driven by Mr Adam for a search at a checkpoint on the Asian Highway in tambon Wisaitai of Chumphon's Sawi district.
They found a black suitcase containing 66,000 methamphetamine pills in the boot of the car.
Mr Adam said he had delivered the drugs from Bangkok to a recipient in Songkhla, but the customer refused to accept them, saying they were of poor quality. He was taking the drugs back to Bangkok to get replacements, only to be caught in Chumphon.