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Tess Vigeland: This week, we're featuring a grudge match. A catfight! A 15-round pay-per-view battle to the bell! Over index funds. In one corner, Fidelity Investments. In the other, Vanguard. Fidelity is trying to beef up its share of the business and that growth could come at the expense of Vanguard. All this over something that doesn't usually generate a whole lot of money. We asked Marketplace's Nancy Marshall Genzer to put her finger on the index fund wars.

Nancy Marshall Genzer: (Dueling banjo music plays in the background) It started quietly enough. Fidelity slashed the fees on its index funds four years ago. Since then, Fidelity has nearly doubled the money in its index funds to about $85 billion. That's still dwarfed by the $600 billion in Vanguard's index funds. So, Fidelity took out full-page ads recently. The headline: Attention Vanguard Index Fund Owners: Fidelity has index funds with lower expenses.

Dan Culloton: Part of it is, you know, to hit a competitor, a fierce rival, where it hurts. You know, saying, "Oh, you say you're the low-cost leader in mutual funds? We offer lower index funds. So there."

Dan Culloton is a senior mutual fund analyst with Morningstar. He says Fidelity and Vanguard aren't dueling over crumbs. Even low fees can add up if you have enough customers. And index funds are easy. They just follow an index -- say the S&P 500. Computers do a lot of the work. Dan Wiener edits the Independent Adviser for Vanguard investors.

Dan Wiener: You don't need to have a big research department. You don't need analysts speaking to corporate management, digging into the financials.

But Fidelity isn't getting rid of the high-priced experts who manage its traditional mutual funds. After all, that's where most people have their money invested. Morningstar's Dan Culloton says Fidelity could be using index funds as a loss leader.

Culloton: You know, an attempt to get new investors in the door, you know, to sample its other financial services offerings, its other mutual funds.

Of course, those other funds have higher fees. Will the new Fidelity campaign work? I trekked out to the suburban Washington office of financial planner Steve Weisman to find out.

Steve Weisman: (Speaks to someone in his office.) We're going to have a new client coming in this morning.

Weisman told me Fidelity might pick up new customers, but from brokerage houses.

Weisman: So, it might be people who were with Merrill Lynches, the Smith Barneys, the UBS's, Fidelity will pick up its share from that.

(More dueling banjos) Weisman says Vanguard investors are loyal. They won't sell the company down the river. And Vanguard isn't about to let its customers float away. So, the dueling will continue, although it may end in a draw.

I'm Nancy Marshall Genzer, for Marketplace Money.

(Banjo music ends. Musician says, "You wanna play another one? Give him a couple bucks.")