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At first glance, the numbers behind the growth in exchange-traded funds and notes can appear mind-boggling: nearly $1 trillion in assets, more than 1,100 ETFs and ETNs in the marketplace, more than 40 companies manufacturing and marketing the products.

But the most startling number could be one of the smallest: three. That's how many firms continue to dominate the industry.

BlackRock (BLK, news), which owns the iShares brand of ETFs, State Street (STT, news) and Vanguard together control 83% of the market, according to the National Stock Exchange, which tracks ETF data.

That's virtually identical to the amount they controlled a year ago, despite six new rivals and dozens of new ETFs and ETNs issued in 2010.

The concentration has made a few industry watchers uneasy, with concerns that having nearly all the money with just a few companies could contribute to higher costs for investors.

"The more assets stick, the easier it is to resist pressure on pricing," says Standard & Poor's equity analyst Tom Graves.

What's unusual, some experts say, is that it can be easy to think of exchange-traded funds and notes as commodities, where brand names shouldn't matter much.

After all, both ETFs and ETNs bundle stocks, bonds or other assets into securities that people can then trade as a stock. Most ETFs and ETNs cost less than actively managed mutual funds. They even have three-letter ticker symbols.

But clearly, some investors are paying attention to the names behind the funds.

A whopping 36% of the $113 billion in new money that investors threw into exchange-traded products in 2010 flowed to funds run by Vanguard, more than went to any other ETF producer.

The Malvern, Pa., company now controls 15% of the assets invested in exchange-traded products, according to the National Stock Exchange.

It's an odd twist for Vanguard, which for years shunned the products and now wholly embraces them.

For their part, the big ETF players assert that investors have plenty of reasons to pick their funds. iShares says competition within the industry has brought investor costs down, while Vanguard says its average ETF expenses are far lower than the industry's average.

And when it comes to competition, investors are in a better position than they were a few years ago, when there were just iShares and State Street, says Morningstar analyst Scott Burns.

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"When fund companies compete, investors win," Burns says.

One group, however, still isn't settling for the status quo: other asset management companies. T. Rowe Price Group (TROW, news) and other heavyweights are expected to introduce ETFs this year, giving investors even more options.

This article was reported by Ian Salisbury for SmartMoney.