By Anjani Trivedi 

In the months before Pacific Investment Management Co. founder Bill Gross quit in September, the firm had increased its bullish bets on the Chinese economy.

The Newport Beach, Calif., money manager doubled down on its position in the second quarter by selling insurance-like contracts to investors that pay out when a country or a company defaults on its debts, essentially wagering that China would continue at its strong pace of economic growth.

Pimco benefits if the price of government debt rises, while buyers of these contracts take the opposite view and wager on a decline in bond prices or a default. In addition to offering a way to take positions on Chinese debt, the contracts--called credit-default swaps--provide a means to bet on the Chinese economy, as foreign investors are limited on how much they can invest in Chinese stocks and bonds. They are also used by some investors to hedge positions in portfolios of Chinese corporate bonds.

As of June 30, Pimco had sold $6.8 billion of credit-default swaps on Chinese government debt, according to the most recent securities filings that include the data. Most of the positions were held in Pimco's flagship fund, the Total Return fund, which had been managed by Mr. Gross and is the biggest bond fund in the world by assets. As of June 30, the Total Return fund had sold about $4.4 billion of these swaps, according to filings.

Representatives for Pimco and Janus Capital Group Inc., Mr. Gross's current firm, declined to comment. Mr. Gross left to join Janus after he found out that other executives at Pimco planned to fire him.

As concerns have emerged about the slowing Chinese economy, the world's second largest, the market for credit-default swaps on China's government debt has doubled to $14.1 billion over the past year and now ranks third in the world behind Italy and Brazil, according to data from the Depository Trust and Clearing Corp. and the International Swaps and Derivatives Association. Different accounting and disclosure practices, though, make it difficult to precisely gauge the market.

The market for Chinese government bonds denominated in foreign currencies is small. Credit-default swaps offer a way to make bigger bets on the debt than would otherwise be possible. Selling credit-default swaps is appealing because returns can be much higher than from buying the foreign-currency government debt.

Pimco's $6.8 billion bet appears to be the biggest position in the market, dwarfing the roughly $1.5 billion in outstanding dollar-denominated Chinese government debt. It is unclear from more recent reports on Pimco's website whether the fund has since changed its position.

China has never defaulted on its government debt. Investors said the chances of the government defaulting are remote because the country has huge reserves of foreign exchange and little foreign debt.

The credit-default-swaps market allows investors "to take exposure or hedge positions elsewhere beyond what would be possible in just the relatively small physical [bond] market," said Mohammed El-Erian, chief economic adviser of Pimco parent Allianz SE and the former CEO of Pimco. Mr. El-Erian, Mr. Gross's former heir apparent, left Pimco in January after butting heads with Pimco's founder.

Mr. El-Erian declined to comment on Pimco's credit-default-swaps positions.

Credit-default swaps are used by investors, hedge funds and others to make bets on sovereign or corporate debt, or wager on a company or country. Credit-default swaps on the subprime-mortgage market were behind the 2008 bailout of insurer American International Group Inc.

Over the past couple of months, the price of five-year contracts for credit-default swaps on China have swung from a nine-month low on Sept. 5 to a seven-month high on Oct. 16, making the country appear riskier at times than Spain and other struggling economies. At one point in October, when it cost about $87,000 a year to insure $10 million of Chinese debt for five years, five-year protection on Spanish debt cost $78,000 to insure the same amount.

Credit-default swaps on China are volatile, reflecting fast-changing sentiment as Beijing seeks to manage slowing growth, high debt loads and a transitioning economy. From declining growth last quarter--the slowest in five years--to falling home prices, the past few weeks have seen a number of gloomy reports out of China.

Pimco sold $4 billion of credit-default swaps on China in the second quarter, as worries about China's growth prospects intensified.

Some investors buy credit-default swaps to hedge a Chinese corporate bond portfolio, said Arthur Lau, head of fixed income for Asia, excluding Japan, at PineBridge Investments, which manages $73.8 billion. "But given that China sovereign default is unlikely, it may not make much economic sense to have a net long China sovereign [credit-default-swaps] position for a long period of time," he said. Mr. Lau said some of his firm's funds use credit-default swaps to hedge their Chinese portfolio positions.

Other investors, though, said because these swaps are designed to insure against default of a sovereign bond, using them to hedge positions in China's corporate debt market isn't effective.

Ken Hu, chief investment officer for Asia-Pacific fixed income at Invesco Ltd., which has $789.6 billion under management, said, "Sovereign [credit-default swaps are] the least attractive [hedging option], because you're hedging for the wrong scenario." Mr. Hu said there are many other ways to hedge risks in a China bond portfolio, including options in stocks and buying onshore Chinese government bonds.

Neeraj Seth, BlackRock Inc.'s head of Asian credit, said his funds prefer to use options on stocks to hedge bond portfolios. He said his funds will sometimes use credit-default swaps to broadly bring down overall macroeconomic risk in their portfolios.

Write to Anjani Trivedi at anjani.trivedi@wsj.com