With the mutual fund investigation now extending to the issue of fund fees, Hewitt Associates issued a warning to employers offering 401(k) plans: Understand your fees.

The company warns that ineffective handling of 401(k) plans, especially investment management, administration and trustee costs, could lose millions for the plan and cut potential retirement earnings by hundreds of thousands of dollars.

"What many employers don't fully realize is that as assets in the 401(k) plan grow over time, so does the total plan cost," said Pam Hess, a defined contribution consultant at Hewitt Associates.

Hewitt suggests that employers take the time to hook up with their 401(k) providers and analyze costs and fees to make sure it benefits the employees. What employees can do, the company said, is periodically ask human resource departments to review plan costs.

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