Counter staff at nine post offices are to go on a half day strike later on Friday.
Royal Mail depots have been picketed by striking workers
It will be the fifth walkout in a continuing dispute over pay and conditions.
The nine post offices affected are in Newtownards, Bangor, Antrim, Coleraine, Londonderry, Lisburn and two in Belfast.
The strike will begin at 1430 BST. The Post Office said it would run a normal service at the branches.
Johnny McKeever from the Communication Workers Union said strikes were set to continue, with the next one planned for Tuesday.
He said counter staff would come off worst under the new pay and restructuring plans.