It's all but inevitable that the Vanguard 500 Index Fund will soon dethrone the Fidelity Magellan Fund as the world's largest mutual fund.

That will provide some of the plainest in-your-face evidence yet of the ascendancy of stock index funds against their actively managed competitors, which have had trouble keeping up with the Standard & Poor's 500, the money manager's benchmark.

Unlike managed funds, index funds make no attempt to pick winning stocks. They aim simply at duplicating the performance of a market measure such as the S&P 500.

Magellan, the giant of the business since the days of legendary manager Peter Lynch a decade ago, surpassed $100 billion in assets last month. Vanguard 500, with close to $93 billion at midyear, is in hot pursuit. Its assets have grown by more than 10 times in the past five years, from $8.4 billion as of mid-1994.

Just three years ago, Fidelity Magellan, with assets of $54.5 billion, was more than twice as big as Vanguard 500, with $23.7 billion.

Still, amid the hoopla that is bound to surround a change at the top, there are several buts to keep the occasion in perspective.

First, Vanguard 500 would have knocked off Magellan long ago if the sponsoring Vanguard Group, in Valley Forge, Pa., hadn't created an almost identical fund in 1990 aimed at big investors. As of June 30, the Vanguard Institutional Index Fund had $26.6 billion in assets that otherwise presumably would have been invested in the 500 Index fund.

On the other side of the coin, Magellan surely would be bigger than it is today if executives at Fidelity Investments in Boston hadn't closed the fund to most new investors in September 1997.

The closing was seen as an important step to help a new manager, Robert Stansky, get Magellan's performance back on track after a spell of subpar results.

Stansky has had success. Magellan beat the Vanguard 500 by 5 percentage points in 1998, with a 33.6 percent return. This year, through Aug. 3, Magellan has gained 9.6 percent to Vanguard 500's 8.8 percent.

"Stansky is a very high-quality manager," said John Rekenthaler, research director at Morningstar Inc., a Chicago-based fund-tracking firm. "It's tough to run an actively managed fund at $100 billion." Still, Rekenthaler said it's inevitable that Magellan soon will fall to No. 2, and "I think it stings a little."

Fidelity executives insist they care far more about keeping Magellan's performance strong than being No. 1 in assets. The company watches over Magellan so solicitously that it has always declined to "clone" it. Cloning is a common industry practice with successful funds, in which the managing firm launches a separate but very similar fund.

Within Fidelity there are several clones, including Equity Income II and Growth and Income II.

Vanguard cloned one of its most popular managed stock funds, Windsor, by creating Windsor II in 1985. In that case, the clone has surpassed the original. At mid-1999, Windsor II had $35.27 billion in assets, almost twice the $18.85 billion for Windsor, which has suffered through a performance slump the past two years.

As close in size as their two largest individual funds are, Fidelity overall remains much larger than Vanguard. As of midyear, Financial Research Corp. in Boston reported assets of all Fidelity funds at $717 billion, compared with $516 billion for Vanguard.