Six hundred workers have gone on strike at the Alexander Dennis bus factory in Falkirk - the first of two stoppages.
The Unite union has rejected a 3.5% pay offer - saying it was below expectations at a time when profits and executive pay were rising rapidly.
Bosses earlier suggested they may have to move production away from the Camelon base as a result of the action.
A senior ADL figure added the company would "have to look seriously at where we build vehicles in the future".
Unite has been pressing for a 4% pay rise but the company has offered 3.5%.
