75 employees of the 2 Sisters poultry processing plant in Anglesey have tested positive for coronavirus.
More than 350 members of staff have been tested so far and the number of cases is expected to grow as more tests are processed.
Employees have been asked to self-isolate for 14 days to help limit the spread of the virus in community.
Production at the plant, which has contracts with Marks & Spencer and KFC, has been shut down for more than a fortnight.
A testing facility has been set up in Llangefni, and an existing facility in Bangor is also being used.
A further facility has also been set-up in Holyhead to test all staff.