Four men have been found working illegally in an abattoir after a raid, says the UK Border Agency.
The men, from Pakistan and South Africa, were discovered at Sher Foods on the Cibyn Industrial estate in Caernarfon, Gwynedd.
The agency said three of the men were discovered after they checked on a 40-year-old man from Pakistan.
A spokesman for the company said it had not set out to employ illegal workers.
The UK Border Agency said the 40-year-old man had been working illegally because his visa had expired.
A search of caravans, used as accommodation by workers in the abattoir's car park, resulted in the discovery of three more offenders, said the agency.
They were a 33-year-old from Pakistan and two 40-year-olds from Pakistan and South Africa.
The 33-year-old and one of the 40-year-olds were arrested and remain in detention pending their removal from the UK.
The South African, and a man who worked as a cleaner, were placed on immigration bail.
They must report weekly to a local police station while arrangements are made to deport them.
If the employer is unable to provide evidence that legally-required checks were carried out before giving the offenders jobs, a fine of up to £10,000 will be imposed for each illegal worker.
David Griffiths, head of the UK Border Agency's north Wales local immigration team, said more operations to tackle immigration offenders were planned.
"Illegal working has a serious impact on communities, taking jobs from those who are genuinely allowed to work," he said.
"Business which operate outside the law should be warned that they will be found and they will be punished," he added.
Arfan Rafique, from Sher Foods, said: "From our side no-one goes out to employ immigration offenders.
"I feel awful for these men, because I know them, and they are good workers.
"They do this kind of thing out of desperation," he added.