By Gregory Zuckerman, Katy Burne and Sarah Krouse 

The Greek government is working on a bailout, Chinese stocks are slumping and U.S. investors are counting down to an expected interest-rate increase.

But what is really concerning a growing number of investors is whether a crisis is brewing in the expanding world of exchange-traded bond funds.

Bond ETFs have emerged as one of Wall Street's most lucrative niches in recent years, promising buyers the steady income of bonds in a package that is as easy to trade as stocks. Now, a building chorus of investors--including activist investor Carl Icahn--is warning that this best-of-all-worlds pitch may be a mirage. They argue that troubles could arise if the bond market has a sharp showdown, perhaps due to higher interest rates, and investors in ETFs start heading for the exits.

So far, most ETFs have held up nicely and investors have continued to shift money to the ETFs, which trade on an exchange and typically track an index or other basket of assets. And the hand-wringing partly stems from investors' painful memories of 2008's market collapse and their vigilance over any emerging dangers.

But at a hedge-fund conference in New York on Wednesday, Mr. Icahn said some ETFs have bought so many riskier and infrequently traded bonds that it isn't clear who will buy them, or at what price, should the funds be forced to sell during a market panic.

In doing so, he joins skeptics who say ETF selling could spread the effects of a downdraft, because the ETFs are widely held by individual investors, who often flee in a market downturn.

Mr. Icahn made his comments in a confrontational session with Laurence Fink, chief executive of BlackRock.

Mr. Fink said Wednesday on an earnings call with analysts that the funds added to market liquidity in part because buyers and sellers of ETFs interact with each other without affecting the underlying basket of assets.

Last month, Bill Gross, who helped found bond giant Pacific Investment Management Co. and now runs a fund for Janus Capital Group Inc., sent a note to investors bemoaning the lack of liquidity and how "mutual funds, ETFs, and even index funds" might be hit in a downturn.

"The obvious risk--perhaps better labeled the 'liquidity illusion'--is that all investors cannot fit through a narrow exit at the same time," Mr. Gross wrote.

One leading high-yield ETF, BlackRock Inc.'s iShares iBoxx High Yield Corporate Bond ETF, traded more than 20,000 times on average each day, according to the firm. But each of its top 10 bonds only traded 13 times a day on average.

High-yield ETFs had their biggest inflows on record in the week ended Wednesday, taking in $1.5 billion, according to fund tracker Lipper.

The iShares fund, which trades under ticker "HYG," saw $1 billion of that cash.

"The reality is that the ETFs are really only as liquid as their underlying assets," said Deborah Fuhr, founder of London-based exchange-traded product consultancy ETFGI LLP.

"You can trade small amounts on a secondary basis, but the real liquidity, when it comes to sizable trades, is the liquidity of the underlying" bonds she added.

Mike Buchanan, deputy chief investment officer at Western Asset Management, said longer-term investors needed to consider that despite "minute-by-minute" liquidity in ETFs themselves, the underlying asset class, particularly fixed income, rarely trades so frequently. "There's a bit of a mismatch there," he said.

Bond ETFs now own 10.5% of U.S. taxable bonds, a record, up from 8.26% two years ago.

All told, investors have put more than $100 billion into fixed-income ETFs over the past 18 months ended in June, according to the fund tracker Lipper.

BlackRock hosted a conference call for clients on Thursday with senior executives on fixed-income market liquidity. It included a discussion on how fixed-income trading and market structure has evolved since the financial crisis and how bond ETFs can be used to invest in fixed income alongside traditional investments.

A BlackRock spokeswoman said in a statement Thursday: "In recent periods of market stress, ETFs have been adding liquidity beyond what's available in the underlying bond market. They have been tested and performed. That's why, rather than part of the problem, they are part of the solution."

The Bank for International Settlements in June highlighted an increased reliance on bond mutual funds and ETFs by investors as a "key change" in fixed-income markets.

"The growing size of the asset-management industry may have increased the risk of liquidity illusion: Market liquidity seems to be ample in normal times, but vanishes quickly during market stress," the BIS wrote.

The Securities and Exchange Commission in June asked for public comment on new and complex exchange-traded products.

As part of the review of how products are listed and traded, the SEC is seeking feedback on products investing in less-liquid assets "including fixed-income instruments" in times of market stress.

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