Farm contractors have unearthed 2,000 Roman coins beneath a field at a farm near Carmarthen.
Coins of the 'late Roman' period feature the standard 'head' and 'tails'
The coins, which date from late Roman times, have been categorised as "treasure".
They contain a small amount of silver, but experts have not yet put a value on the find.
The exact location of the discovery is being kept secret to protect the site from treasure hunters. The HM Coroner has been informed.
The coins are thought to have been lying just 12 inches beneath the surface of a field.
The Romans left Wales in 410AD, having first arrived in 47AD. Carmarthen was a Roman settlement from the first century AD.
Coins of the late Roman period feature the standard head and tails, with a Roman emperor on the head and a goddess - of fortune, hope or chastity - on the reverse.
The farmer, on whose land the find was made, said: "There are lots of wild rumours flying about but I don't really want to say anything.
"The coins are at the museum now, and we are waiting to hear what happens next," he added.
Edward Besley, of the National Museum of Wales, in Cardiff confirmed that the coins were at the museum and were being cleaned, examined and catalogued.
He said: "When our investigations are complete, I will submit a report to the coroner who will then decide when to hold an inquest."