By Min Zeng

Bill Gross, manager of the world's biggest bond fund, cut slightly both holdings of Treasury bonds and mortgage-backed securities in July.

Treasury bonds holdings at the $270 billion Total Return Fund (PTTRX), managed by Mr. Gross, who is co-chief investment officer at Pacific Investment Management Co., slipped to 33% last month from 35% in June, according to data released late Thursday afternoon on the company's website.

Meanwhile, the fund's holdings of U.S. mortgage-backed securities ticked down to 51% from 52% at the end of June, unchanged from May.

Still, Treasury bond and MBS holdings accounted for 84% of the fund. Mr. Gross have heavily parked cash in high-grade U.S. assets this year, a reflection of his worries over the global economic outlook stung by the euro zone's sovereign debt crisis.

Like some other investors, Mr. Gross has argued that interest rates in the U.S. would remain low and could even go lower thanks to the Federal Reserve's easy monetary policy.

Speculation has risen that the Fed might launch a new bond-buying program in September targeting both MBS and longer-dated Treasury bonds--which could boost the value of Mr. Gross's bond fund.

Mr. Gross's fund has handed investors a return of 7.3% this year through Wednesday, beating 92% of comparable bond funds and outperforming the 3.3% return on the benchmark Barclays Capital US Aggregate Bond Index.

Over the past 15 years, the fund has handed investors an annualized return of 7.4%, compared to 6.3% on the benchmark index. Pimco, part of Allianz SE (ALV.XE, ALIZF), is one of the world's biggest asset-management companies, with over $1.7 trillion in assets under management.

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