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When it comes to high-risk bonds, the asset management giant Pimco has pretty much cornered the global market.

Be it bonds issued by the automotive financier Ally Financial or the student loan financier SLM in the United States, or government bonds in Spain and Italy, Pimco holds a commanding position in these high-yielding securities.

But as Pimco’s portfolio managers double down on their bet that high-risk bonds will thrive in a world of low interest rates, a growing number of global regulators are warning that the positions being taken on by the big asset management firms pose a broad danger to the financial system.

These concerns were amplified this week as stock markets gyrated, the yields of high-risk corporate and European bonds spiked upward and, crucially, trading volumes evaporated.

Regulators and bank executives have cautioned that an accumulation of hard-to-trade, risky bonds by a small group of fund companies could turn a bond market hiccup into a broader rout, in light of how illiquid many of these securities have become.

Junk Bond Investors Pimco is one of the largest managers of high-yield, or junk, bonds. But recent jitters in financial markets have raised concern about these types of investments, which can be risky and difficult to trade. Asset manager’s stake of the company’s high-yield debt Asset manager Debt issuer 1. Pimco Ally Financial 37.3 % 2. Pimco SLM 29.9 3. Pimco A.I.G. 26.4 4. Franklin Templeton First Data Corp 25.5 5. Franklin Templeton Tenet Healthcare 23.4 6. Franklin Templeton Caesars 18.3 7. Pimco CIT Group 18.0 8. Franklin Templeton Charter 13.5 9. Franklin Templeton Sprint 12.6 10. Franklin Templeton Chesapeake Energy 11.8 11. Franklin Templeton Reynolds Group 11.6 12. Franklin Templeton Community Health Systems 11.2 13. Pimco HCA 11.1 14. JPMorgan Chase Dish Network 10.1 15. Pimco MGM 9.7 Asset manager’s stake of the company’s high-yield debt Asset manager Debt issuer Pimco Ally Financial 37.3 % Pimco SLM 29.9 Pimco A.I.G. 26.4 Franklin Templeton First Data Corp 25.5 Franklin Templeton Tenet Healthcare 23.4 Franklin Templeton Caesars 18.3 Pimco CIT Group 18.0 Franklin Templeton Charter 13.5 Franklin Templeton Sprint 12.6 Franklin Templeton Chesapeake Energy 11.8 Franklin Templeton Reynolds Group 11.6 Franklin Templeton Community Health Systems 11.2 Pimco HCA 11.1 JPMorgan Chase Dish Network 10.1 Pimco MGM 9.7

The latest to sound the alarm was the International Monetary Fund, which recently released a report warning about large bond investors like Pimco that have built up dominating stakes in high-risk, hard-to-sell bonds all over the world.

“Credit-focused mutual funds have seen massive inflows and have become the largest holders of corporate and foreign bonds,” José Viñals, the head of the I.M.F.’s financial-markets division, said at a news conference in Washington last week. “These inflows have created a liquidity illusion, which can amplify shocks and lead to sharper falls in the market.”

The issue has also drawn the attention of the Federal Reserve Bank of New York.

In its paper, the I.M.F. highlighted high-yield bonds issued by Ally Financial, SLM and the insurance giant American International Group. Pimco owns at least 25 percent of the outstanding debt of each of those three companies.

Outside the United States, the numbers were even starker: Pimco owned close to 50 percent of a number of foreign bonds. And it controlled over 40 percent of the debt issued by the Bank of China, just under 40 percent of the State Bank of India and close to 30 percent of the Spanish bank BBVA.

Of course, Pimco has company in accumulating outsize positions in these and other companies.

As the I.M.F. report lays out, Franklin Templeton Investments has large stakes in the high-yield bonds issued by Tenet Healthcare and the payment processor First Data, among others. Franklin has also bet big on bonds from Ireland and Ukraine. According to the fund, managers like Fidelity Investments, Dodge & Cox and BlackRock have significant, though smaller, positions in selected debt issues.

That asset management companies have emerged as the main providers of riskier types of credit to companies around the world is a little-appreciated consequence of increased regulation of banks following the financial crisis.

With banks shying away from these types of loans, bond-oriented mutual funds — which have benefited from an asset boom in the last five years — have stepped up.

For the first time ever, according to I.M.F. data, mutual funds have surpassed banks as the largest holders of corporate and foreign bonds, holding 13 percent of these securities.

Before 2008, for example, investment banks like Goldman Sachs and JPMorgan Chase were the main buyers and sellers of these bonds, earning profits for themselves and making a market for their clients.

Now, with banks being pressured to take fewer risks, they no longer trade or hold these securities in significant amounts. Wall Street executives worry about how the markets will react if funds start to sell off their concentrated positions.

The chief risk officer of Goldman Sachs, Craig W. Broderick, warned at the I.M.F. meetings last week that the asset management firms that now hold the bulk of these bonds had not yet been tested in terms of how they would react to a market shock.

“Now there is plenty of liquidity,” Mr. Broderick said. “But when things are different, the alternative providers will not be there.”

The term of art for this scenario is a liquidity mismatch, with some going so far as to call it a systemic liquidity mismatch. If, for example, there is a sustained emerging-market crisis and a fund wants to liquidate these bonds to meet redemption demands, the manager will be required to provide cash immediately even though it may take several days to sell the securities in question.

Traders calculate that less than 1 percent of corporate bonds trade more than $5 million a day.

“People are worried about massive liquidations in a market that is not as liquid as it used to be,” said Amy Koch, a senior trader at Standish, a Boston-based bond manager.

The debate about whether asset management companies pose a broad risk to the economy has been fairly narrow, confined to several academic papers and a few hearings in Congress.

The notion that fund companies might be lumped in with big banks in terms of risk has also been opposed very forcefully by the industry, which argues that unlike banks, asset management firms do not use debt to enhance their returns and do not put any of their own cash on the line.

But the abrupt departure last month of William H. Gross from Pimco, the firm that he made famous, has focused attention on the issue — and on Pimco in particular.

Pimco oversees about $1.9 trillion in bonds — making it the largest bond manager in the world. And unlike its peers, it invests all this money on the basis of a firmwide investment view that was heavily promoted by Mr. Gross.

Especially vulnerable, I.M.F. economists say, are companies in which one manager and one investment view hold sway over a wide family of funds. That can lead to a situation in which numerous funds companywide accumulate concentrations in the debt of a certain company, sector or country. When retail investors are driving the investment money coming in and flowing out, the dangers are compounded.

To some extent this thesis was put to a test — which Pimco seems to have passed — following Mr. Gross’s hasty exit last month. Although small investors yanked billions of dollars out of the Total Return Fund, the flagship vehicle managed by Mr. Gross, there was no sign of dislocation in the markets.

And analysts point out that Pimco’s so-called house view served the firm well during the financial crisis of 2008, and it also made an early and successful call that eurozone bonds would fall in 2010 and 2011.

Still, over the last 10 months, investors have pulled $65 billion out of Pimco — $42.8 billion from Total Return and the rest from other bond funds, according to Morningstar.

If redemptions persist and bond trading conditions worsen, Pimco would be forced to unload some of its larger positions. But selling more than 40 percent of the outstanding bonds in Qatar National Bank or over 20 percent of the debt of the Hong Kong gambling company Melco Crown Entertainment could be a difficult proposition, given how little these bonds trade.

Pimco declined to comment for this article.

Its holdings in Italian and Spanish government bonds are not as concentrated, because the markets for government bonds are much larger.

Still, the bond giant owns over $35 billion in bonds from Italy and Spain that mature from 2015 to 2017. The bet is that securities from these countries, despite their torpid growth and heavy debt load, will be bought up by the European Central Bank — an outcome that has yet to be determined.

“We are worried about liquidity,” said Laurence D. Fink, the chief executive of Pimco’s archrival BlackRock, in a conference call with investors this week. “And the question is, do we have enough time before a true liquidity event destabilizes the market?”

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