The Labrador retrievers are genetically identical
South Korea's customs service says it has deployed the "world's first cloned sniffer dogs" to check for drugs at its main airport and border crossings.
Six puppies cloned from a Canadian-born sniffer dog in late 2007 have reported for duty after completing a 16-month training programme.
The clones are all called Toppy, a combination of "tomorrow" and "puppy".
The customs agency says clones help to lower crime-fighting costs as it is difficult to find good sniffer dogs.
Only about 30% of naturally-born sniffer dogs make the grade, but South Korean scientists say that could rise to 90% using the cloning method.
'World's first cloned sniffer dogs'
The new recruits are part of a litter of seven puppies who were cloned from a "superb" drug-sniffing Canadian Labrador retriever called Chase in 2007, officials said.
One dropped out of the training due to an injury.
"They are the world's first cloned sniffer dogs deployed at work," Park Jeong-Heon, a customs spokesman at Seoul's Incheon International Airport, told AFP news agency.
"They showed better performances in detecting illegal drugs during the training than other naturally-born sniffer dogs that we have."
The cloning work was conducted by a team of scientists at Seoul National University, who created the world's first cloned dog - an Afghan Hound named Snuppy.
The state-funded project cost about 300m won ($239,000; £146,000).