Total U.S. money market mutual fund assets fell $15.49 billion to $2.622 trillion for the week that ended Wednesday, the Investment Company Institute said Thursday.

Assets of the nation's retail money market mutual funds rose $1.81 billion to $914.46 billion, the Washington-based mutual fund trade group said. Assets of taxable money market funds in the retail category rose $880 million to $720.02 billion. Tax-exempt retail fund assets rose $930 million to $194.44 billion.

Meanwhile, assets of institutional money market funds fell $17.30 billion to $1.708 trillion. Among institutional funds, taxable money market fund assets fell $15.72 billion to $1.615 trillion; assets of tax-exempt funds fell $1.58 billion to $92.38 billion.

The seven-day average yield on money market mutual funds was 0.03 percent in the week that ended Tuesday, unchanged from the previous week, said Money Fund Report, a service of iMoneyNet Inc. in Westborough, Mass.

The 30-day average yield was also unchanged from last week at 0.03 percent. The seven-day compounded yield was flat at 0.03 percent. The 30-day compounded yield was unchanged at 0.03 percent, Money Fund Report said.

The average maturity of portfolios held by money market mutual funds was unchanged from the previous week at 46 days.

The online service Bankrate.com said its survey of 100 leading commercial banks, savings and loan associations and savings banks in the nation's 10 largest markets showed the annual percentage yield available on money market accounts was unchanged from the previous week at 0.14 percent.

The North Palm Beach, Fla.-based unit of Bankrate Inc. said the annual percentage yield available on interest-bearing checking was also unchanged from the week before at 0.06 percent.

Bankrate.com said the annual percentage yield on six-month certificates of deposit was unchanged from the previous week at 0.22 percent. The yield was unchanged at 0.34 percent on one-year CDs and rose to 0.53 percent from 0.52 percent on 2 1/2-year CDs. It was unchanged at 1.14 percent on five-year CDs.