Money market mutual funds have become a popular haven over the past two months, even as the interest rates on those funds decline.

The funds pool money from individuals and invest in short-term securities, such as Treasury bills and high-grade commercial debt. They pay interest rates comparable to six-month certificates of deposit. Unlike CDs, however, the rates vary from week to week, and investors can withdraw money at any time, often by writing a draft similar to a bank check.More than $6 billion poured into money funds in the week ended Nov. 21, according to the Investment Company Institute, a mutual fund trade organization. That's the biggest jump since January.

Much of the money came from investment professionals, who may shift the money elsewhere just as suddenly, said institute spokesman John Collins. Nevertheless, individual investors poured more than $2 billion into money market funds last week.

Reason 1: The troop buildup in Saudi Arabia. The threat of war has investors nervous, investment experts say. When times are tense, investors traditionally put their money where they can get at it easily.

Reason 2: Concern about the quality of commercial debt. Much of the new money is pouring into funds that buy government-guaranteed securities, such as Treasury bills, said Mary Ruth Moran, a marketing manager for Fidelity Investments, a Boston mutual fund company.

The funds themselves aren't insured, but their holdings are. That reassures investors, Moran said.

At Fidelity, assets in Treasury money funds have increased 47 percent over the past 3 1/2 months. Assets in funds that buy other government debt grew 27 percent. Other money funds have stayed about even, she said.

Investment companies have taken note, pumping their advertising dollars into promoting their government funds, Moran said.

For the increased safety, investors in government funds sacrifice yield, about one-tenth of a percentage point, compared with money funds on the whole.