Vanguard Group's Extended Market Index Fund will change its target benchmark from the existing Dow Jones Wilshire 4500 to the Standard & Poor's Completion Index.
The switch will affect all four of the $9 billion fund's share classes.
Adoption of the S&P Completion Index for the Extended Market Index Fund will complement the $102 billion Vanguard 500 Index Fund, which has tracked the S&P 500 since 1976, said Gus Sauter, chief investment officer.
The S&P Completion Index represents mid- and small-capitalization stocks and was developed by S&P to complement the large-capitalization S&P 500 Index.
"By holding a combination of the two Vanguard index funds, investors may obtain exposure to the entire U.S. stock market," Sauter said.
The change is expected to occur between May 31 and Dec. 31, 2005.