The costs of investing through financial advisers or brokerages continue to plummet. But there’s one sneaky fee that just won’t disappear—and it could cost you tens of thousands of dollars in returns over your lifetime. When you invest in a mutual fund, you could be sold a share class that tacks on an extra fee in addition to the annual expense ratio. It’s known as a “12b-1” fee. It can range from 0.25% to 1%, which means it could cost you as much as an extra $1,000 annually on a $100,000 investment.

Fund companies say that 12b-1 fees are charged to cover their marketing costs. But the majority of the fee goes right back to the person who sold you the fund shares as a recurring commission. That provides a powerful incentive for your adviser or broker to push you into these high-fee shares even if cheaper shares of the same fund, without the 12b-1 fee, are available.

The practice is not illegal for brokers, but investment advisers registered with the Securities and Exchange Commission are held to a higher “fiduciary” standard. They are required to put clients’ interests above their own. That includes disclosing whether they will receive any revenue-sharing payments when selling you mutual fund shares and suggesting cheaper alternatives.

Self-defense: Check whether the fund shares you own or are considering buying charge 12b-1 fees. To do so, look up a fund at ­Morningstar.com and click on the “Purchase” tab near the top of the page. It will show the different share classes and which, if any, charge 12b-1 fees. Tell your adviser or broker that you want the cheapest share class available. More expensive share classes provide no additional rights or advantages. If you own the shares in a tax-deferred account, you can sell them and buy the cheaper share class or shares of a similar fund without 12b-1 fees. In a taxable account, you’ll need to weigh the potential immediate capital-gains tax bite of selling versus the recurring costs of the 12b-1 fee if you hang onto the shares.