Assets of money market mutual funds fell 0.27%, to $1.2778 trillion in the week that ended Sept. 16, the Investment Company Institute reported.
The decline, from $1.2813 trillion on Sept. 9, was the first in nine weeks.
Money market fund coffers had been swelling as gyrations in the stock market sent investors running for cover.
Conversely, last week's decline in money fund assets came as the market staged a four-day rally, suggesting that cash was being redeployed by investors.
Assets of institutional investors dropped 0.63%, to $495.66 billion. Taxable funds shed $1.84 billion, closing at $450.97 billion; tax-exempt funds were off $1.32 billion, to $44.69 billion.
Assets of funds for retail investors inched up 0.04%, to $782.12 billion. Taxable funds picked up $1.88 billion, to end at $642.54 billion; tax-exempt funds fell $1.55 billion, to $139.57 billion.