The Italian-owned company makes filters for the car market
Workers at a car parts factory are taking part in a 24-hour walkout in a row over work agreements.
Unite said members at Sogefi Filtration at Llantrisant, Rhondda Cynon Taf, are angry over proposals to move back from flexible holidays to fixed periods.
It said an overtime ban was also brought in last week at the firm, which makes paper filters for cars.
The firm said it was disappointed the action was being taken, particularly in the current economic climate.
Managing director Stuart Hobbs said all the action it was taking was to "make the company viable and to protect full-time jobs".
He said: "We want to protect our employees. We have had to reduce our temporary workforce because we saw a massive downturn at the end of the last quarter of the year.
Mr Hobbs said they had had to reduce working hours, from November to December last year, but were now back to normal working hours.
He also said they have started consultation on flexible holidays last September.
Flexible holidays were introduced about five years ago but Mr Hobbs said they had found "that its almost impossible to manage the flexibility along the skills we need to run the business".
"Basically people were choosing to take holiday when we really needed them to work, do we decided to keep the plant running efficiently.
"We need to revert back to fixed holidays - staff have always been involved in these discussions."
Last October, over 50 temporary staff were laid off at the plant, where the union said some 550 people work.
The union said around 400 workers at Sogefi are members of the union.
The 24-hour strike started on Tuesday night and workers are standing on the picket line on Wednesday morning.
The company was formerly called Fram Filters.