SAN FRANCISCO — Google Inc.'s stock will be added to the Standard & Poor's 500 index, a long-anticipated rite of passage that lifted the online search engine leader's recently sagging shares.

Standard & Poor's announced the forthcoming change after the stock market closed yesterday. Google will replace Burlington Resources Inc. in the closely watched barometer March 31. Burlington Resources, a major oil producer based in Houston, is being acquired by ConocoPhillips Inc. in a deal worth about $35.6 billion.

Google's shares gained $1.67 to close at $341.89 on the Nasdaq Stock Market, then soared $30.82, or 9 percent, in extended trading.

Before yesterday's announcement, Google's market value had dropped by nearly 20 percent so far this year, wiping out about $20 billion in shareholder wealth amid concerns about slowing earnings growth and questions about its often cryptic communications with Wall Street.

The inclusion into the S&P 500 provides Google's stock with an immediate catalyst because so many large mutual funds are based on the index's composition. Once a stock is added to the index, money managers typically must buy shares as they readjust their portfolios.

What's more, some conservatively managed funds are permitted to invest only in companies that belong to the S&P 500, so the move should make Google's stock available to a larger pool of investors, said David Garrity, director of research for Hapoalim Securities.

Now that Google will be in the S&P 500, Garrity said some investors who couldn't buy Google's shares before might decide to sell some of their stock in Yahoo Inc. — another Internet icon that is already in the index.

"Some of the benefit that Google gets from being in the S&P could come at Yahoo's expense," Garrity said.

Yesterday gave no inkling of that happening. Yahoo's shares gained $1.08 to close at $31.83 on the Nasdaq, then added another 60 cents, or 1.9 percent, in extended trading.

Joining the S&P 500 also is a symbol of prestige that stamps its members as a blue-chip company.

Google's entrance into the elite club had been considered a foregone conclusion by many investors because its market value already had surpassed many of the nation's best-known companies, including the likes of Boeing Co., Hewlett Packard Co. and Home Depot Inc. — all components of the even more exclusive Dow Jones industrial average.

Despite its stock's recent downturn, Google's market value has remained above $100 billion.

Google is making the leap into the S&P 500 at a very early age.

Founded by Larry Page and Sergey Brin in a Silicon Valley garage just 7 1/2 years ago, Google didn't even become a public company until August 2004. That initial public offering at $85 per share turned Page and Brin into multibillionaires.

The company is thriving too, having earned $1.5 billion on $6.1 billion in revenue last year, adding the financial firepower to expand its payroll to nearly 6,000 employees.