"At smaller companies, the owners . . . are busy with other things," Harris said. "The owner might say, 'I need a 401k. OK, let's talk to my insurance guy.' "

Insurers are often more willing than big mutual fund companies to take on small accounts. Unfortunately, insurers typically offer more expensive investment options. Participants in small plans also can wind up paying proportionately more for various account services, since the costs are spread over fewer people.

One way to see if your plan's fees are reasonable is to look up your company on BrightScope, a financial information company that offers retirement plan ratings. (The site may not have a rating for small plans, however, because of lack of data.) If you register, you also can get a free "personal fee report" that summarizes what you're paying for your investments.

Then take a close look at your 401k disclosure forms. If you're seeing total annual costs for an investment option that exceed 1%, you should do a little digging. You can use a site such as Morningstar to check the expense ratios of comparable funds. (Vanguard typically offers low-cost versions of various mutual funds.) If your plan's option is significantly more expensive, or if the total expenses on any option top 2%, it's time to act. You can:

  1. Put your own money in cheaper options, for now. The Department of Labor warns that "cheaper isn't always better" -- perhaps not always, but often it is. There's no evidence that paying more for an investment guarantees better returns; in fact, the opposite is usually true. Low-cost index funds, exchange-traded funds or target-date funds may be your best choices.
  2. Let your employer know you care about costs. Recruit your co-workers, if you can, to request lower-cost options. You also might volunteer to serve on any committee that evaluates plan providers or investment options.
  3. Suggest automatic enrollment. Automatic enrollment significantly boosts participation in 401k plans, since people have to make the effort to opt out, rather than to sign up. Higher participation means more money in the plan, which means more clout in negotiating with providers. Automatic enrollment isn't always a slam dunk -- it may be too expensive for companies with a generous match or impractical for companies with high turnover. Often, though, tapping people's basic inertia can benefit both them and the plan.
  4. Enlist the top dogs. High earners -- top executives at bigger companies, the owner at smaller ones -- may have contributed more than average workers to the 401k plan. If so, they have more money at stake, and should care even more than the average worker about what happens to it. Send them a link to this article, or print it out and leave it on their desks.
  5. Invest on your own. If your plan continues to reek, you may need to take more of your retirement into your own hands by investing outside the 401k. If your company offers a match, you should contribute enough to the 401k to get the full amount. But you also can contribute up to $5,000 a year to an IRA or a Roth IRA ($6,000 if you're 50 and over). Set up one of these accounts at a low-cost provider, and you'll have a world of investments from which to choose.

Click here to become a fan of MSN Money on Facebook

Liz Weston is the Web's most-read personal-finance writer. She is the author of several books, most recently "The 10 Commandments of Money: Survive and Thrive in the New Economy" (find it on Bing). Weston's award-winning columns appear every Monday and Thursday, exclusively on MSN Money. Join the conversation and send in your financial questions on Liz Weston's Facebook fan page.