BOSTON - For the first time in a decade, new investors will be able to put money into a mutual fund that helped fuel Fidelity Investments' rapid growth in the 1980s and '90s, but has recently seen many retirement-age investors withdraw their cash.

The nation's largest mutual fund company said Monday it will reopen its $44.8 billion Magellan Fund to new investors effective today.

Boston-based Fidelity closed Magellan to new accounts Sept. 30, 1997, after a run of market-beating returns in the 1980s by star money manager Peter Lynch and three lesser-known successors in the 1990s.

While Magellan has enjoyed recent strong returns, its size has dwindled from a peak of $102 billion in 2000 because of the 1997 closure, intended to protect existing investors by preventing the fund from becoming too big and unwieldy to manage.

In recent years, Magellan's investors have crept closer to retirement, when many redeem investments to pay for needs after their working lives.

"In fact, 85 percent of the fund's assets are earmarked for retirement, and the baby boomer generation has now begun to retire and tap those dollars," said Walter Donovan, president of Fidelity's equity division.

Retirement-age clients typically make conservative investments to avoid having a sudden downturn deplete their nest eggs just before retirement.