(27/7 News)  --  One of the largest retirement plan providers in the U.S. is facing a lawsuit over the 401(k) plan it offers its own employees. 

Some current and former Fidelity workers are charging that expensive Fidelity mutual funds monopolize the plan.  They say lower-fee options are available within Fidelity's own offerings and from other providers. 

Fidelity is the 401(k) provider for approximately 12-million workers in thousands of companies.  The company has filed to dismiss the lawsuit, citing its employees have a wide variety of investments to choose from, including low-cost index funds.