--Share of Treasury bonds in Total Return Fund at Pacific Investment Management Co. rose to 30% at the end of January from 26% in December

--Share of mortgage-backed securities fell for seventh month in a row to 37% last month from 42% in December

--MBS still accounts for largest share of the fund

By Min Zeng

For Bill Gross, the main trade during the first month of 2013 was cashing more chips out of the mortgage-backed securities and stocking up on Treasury bonds for the world's largest bond fund, which he runs.

The share of Treasury bonds in the $285.6 billion Total Return Fund (PTTRX) at Pacific Investment Management Co. rose to 30% at the end of January from 26% in December and 23% in November, according to the data released on the company's website Monday afternoon. The Treasury debt holdings include regular Treasury bonds and Treasury inflation-protected securities.

In contrast, the share of MBS fell for a seventh month in a row to 37% in January from 42% in December. MBS still accounts for the largest share of the fund.

Mr. Gross, founder and co-chief investment officer at Pimco, told Dow Jones Newswires in December that he continues to lighten up on MBS, its main trade in 2012 that boosted the fund's performance.

Early in 2012, Mr. Gross's fund placed bets that the Federal Reserve would buy MBS as part of its efforts to support the economy, making these instruments the largest holding in its portfolio. That proved correct when the Fed launched an MBS-buying program in September, lifting prices of these bonds.

In recent weeks, Mr. Gross has emphasized his investment strategy for the new year: stay away from longer-dated Treasury bonds and buy those maturing between five and seven years. He also favors TIPS, and sovereign bonds sold by Italy.

Mr. Gross is worried that the Fed's unconventional monetary stimulus via bond purchases likely will push up inflation in the coming years. In this case, longer-dated Treasurys, in particular the 30-year sector, will fall sharply in price as rising prices eat into the value of the bonds over time.

Instead, Mr. Gross has been buying shorter-dated maturities as these bonds still benefit from the Fed buying and their value is at less risk of getting slammed by the inflation threat.

Like many other investors worried about inflation, Mr. Gross has turned to TIPS to hedge inflation risk. The value of TIPS will be adjusted higher if consumer prices climb.

"Investors should be alert to the long-term inflationary thrust of such check writing" by the Fed, Mr. Gross said in his January investment outlook. "While they are not likely to breathe fire in 2013, the inflationary dragons lurk in the 'out' years towards which long-term bond yields are measured."

Pimco, part of Allianz SE (ALV.XE, ALIZF), is one of the world's biggest asset-management companies, with about $2 trillion in assets under management.

Write to Min Zeng at min.zeng@wsj.com