Workers at eight Tesco stores are taking to the picket lines this morning, in a row over contracts for long-term staff.
Tesco is calling on them to accept a Labour Court recommendation and says all stores will remain today despite the action.
The union involved says the strike is totally avoidable.
Mandate Assistant General Secretary Gerry Light says all workers want is a guarantee that contracts won't be changed without agreement: "They can stop it simply by writing to the union and giving us a guarantee that no changes will be brought about without the agreement of the workers.
"They have been saying for the past number of days that the strike is unjustified, because they actually haven’t made the changes, now on Friday, I wrote to the company and said, well if that is the case and if that will remain the case, simply just write to us and confirm this, they have ignored that letter."