employer mandates employees to enroll until open enrollment

[Guest PamM]

Can an employer require employees who enroll in the medical and dental plans to be on the plan until open enrollment? The employee pays 50% of their premium and 100% for their dependents. The employer will not allow them to terminate coverage until open enrollment. Is this legal? THANKS

[E as in ERISA]

Are the employees' premiums pre-tax? If so, then the employees are also signing up for the 125 cafeteria arrangement when they enroll in the medical and dental plans. They generally have to stay in the plan for the rest of the year.