The riskiest money-market mutual funds will be required to abandon their stable, $1-share value and allow their prices to float under rules adopted by the U.S. Securities and Exchange Commission.

The rules, approved today on a 3-2 vote, conclude a four-year struggle to toughen regulations after a run at one money fund during the 2008 credit crisis brought the $2.6 trillion industry to near-collapse, halted only by a federal backstop. Money-fund managers and other business groups largely opposed the new rules.

The strongest of the measures are reserved for prime money funds, which cater to institutional investors and primarily buy riskier securities, such as commercial paper issued by banks. Instead of a stable price of $1, which means a dollar invested can always be redeemed for $1, prime funds will have to price their shares in a way that will reveal fluctuations. Funds will have two years to comply with the change.