Make a trade now and again? You should know that the mutual funds in your plan may disallow or penalize certain actions; your plan administrator, with your employer's input, may have set other rules.
Check with HR to find out what applies to you, but beware these often-restricted moves:
Making a "roundtrip" transaction
The rule: Funds or administrators may limit your ability to sell out of a fund and then buy back in within a short period, called a roundtrip.
For example, the first time you sell a Fidelity fund and then reinvest more than $1,000 into the same fund within 30 days, you'll get a warning; subsequent roundtrips result in restrictions on how often you can trade in the future.
(The Fidelity plan at Time Inc. -- MONEY's parent company -- limits employees to one trade a quarter after the third roundtrip.)
The reason: Roundtrips require managers to buy and sell assets, and therefore hike up administrative costs for the fund, says Mike Alfred, co-founder of BrightScope, which ranks 401(k) plans.
Selling soon after you buy
The rule: With certain funds, if you sell before owning for a minimum period -- usually 30 or 60 days -- you'll pay a fee of up to 2% of the price of the shares.
The reason: Redemption fees are often levied on funds with holdings that are not as easily bought or sold, such as small-cap stocks and international equities, says Susan Powers, senior VP of investment consulting at Fidelity. The lack of liquidity could result in such funds taking a big performance hit as a result of short-term trading.
Buying too much company stock
The rule: Public companies often put a cap on how much employer stock that workers can own, says Powers. The restriction is usually built into the plan, so that you wouldn't be able to invest more than, say, 25% of your money with your company.
The reason: In a post-Enron world, says Powers, 401(k) plans are designed to prevent employees from holding 100% of their portfolio in their employer's stock.