Bird flu has been confirmed in chickens and ducks on a premises near Pontyberem, Carmarthenshire, the Chief Veterinary Officer for Wales said.
The decision was taken to cull the birds before confirmation, amid strong suspicion of avian influenza H5N8.
A 3km protection zone and 10km surveillance zone have been put in place to limit the risk of the disease spreading, and as part of the wider surveillance and disease control measures.
An avian influenza prevention zone declaration was made by Welsh ministers on December 6 last year.
It means extra biosecurity measures for all poultry and captive birds to protect them from the risk from wild birds.
The zone covers the whole of Wales and is due to remain in place until January 6.
It requires the immediate and compulsory housing of domestic chickens, hens, turkeys and ducks, or where this is not practical, their complete separation from contact with wild birds.
For farmed geese, game birds and other captive birds, keepers should take practical steps to keep them separate from wild birds.