Sept. 9 (Bloomberg) -- Salesforce.com Inc. and Fastenal Co. will replace Fannie Mae and Freddie Mac in the Standard & Poor's 500 Index after shares of the biggest providers of U.S. mortgage financing fell below $1 apiece.

Salesforce.com, the largest seller of Web-based customer- management software, and Fastenal, the biggest U.S. retailer of nuts, bolts and other fasteners, will join the benchmark index for U.S. stocks, according to a statement posted on S&P's Web site today. Fannie and Freddie have fallen more than 97 percent this year and their market value slipped under $1 billion yesterday after the government said it would take them over.

The addition of a company to the S&P 500 may support its stock price as money managers tracking the index purchase the shares. CF Industries Holdings Inc., the last company to be added to the S&P 500, rallied for five days after the announcement of its addition on Aug. 22, gaining 5.5 percent. Invesco Ltd. climbed 9.5 percent for the biggest gain in three years on Aug. 14, a day after S&P announced its inclusion.

Fannie Mae and Freddie Mac will be dropped from the S&P 500 after the close of trading on Sept. 10, and Salesforce.com and Fastenal will join after U.S. markets close on Sept. 12.

To contact the reporter on this story: Eric Martin in New York at emartin21@bloomberg.net.

Last Updated: September 9, 2008 17:45 EDT