Besides the mutual fund management fees that 401(k) investors pay, some are being charged hidden commissions, MarketWatch reports, calling it an "insidious form of surcharge."

And rather than levy these fees as cash, which would make them detectable as a line item on a 401(k) statement, plan providers frequently deduct mutual fund shares from an investor's account every quarter. Given that investors automatically contribute to their 401(k) plan every pay period, and balances are constantly rising, this makes it virtually impossible for a 401(k) investor to keep tabs on the total number of shares they own.

Since employers do not pay these fees, they aren't likely to inquire about them. To find out whether or not their 401(k) plan charges these fees, MarketWatch encourages investors to ask their company's benefits department.

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