The riskiest money-market mutual funds will be required toabandon their stable, $1-share value and allow their prices tofloat under rules adopted by the U.S. Securities and ExchangeCommission.

The rules, approved today on a 3-2 vote, conclude a four-yearstruggle to toughen regulations after a run at one money fundduring the 2008 credit crisis brought the $2.6 trillion industry tonear-collapse, halted only by a federal backstop. Money-fundmanagers and other business groups largely opposed the newrules.

The strongest of the measures are reserved for prime moneyfunds, which cater to institutional investors and primarily buyriskier securities, such as commercial paper issued by banks.Instead of a stable price of $1, which means a dollar invested canalways be redeemed for $1, prime funds will have to price theirshares in a way that will reveal fluctuations. Funds will have twoyears to comply with the change.