A benign invasion from Boston has recently given area money market mutual fund players new opportunities and vehicles for creative, self-directed investment. Among the Northeast region's most popular money market funds, Magellan Fund, recently reactivated after a 15-year hiatus and administered by Fidelity Management & Research Co., has taken hold with investors here.

And the Boston-based investment advisory firm's first new equity-based mutual fund, Fidelity Select Portfolios, has caused a ripple of excitement among investors "who can follow market leadership from sector to sector of the economy," according to Fidelity executives.

In Washington itself, Fidelity has about 4,600 accounts with total assets of approximately $65 million, and in the Delmarva peninsula region, another 12,000 accounts valued at $100 million.

The idea behind Fidelity Select Portfolios, says executive vice president John F. O'Brien, is to allow "shareholders to pursue the potential benefits of 'market sector investing' through the convenient and economical vehicle of a no-load mutual fund." The portfolios are currently available in four market sectors: precious metals and minerals, technology, health care, and energy. As other sectors of the market become more attractive to Fidelity's analysts, additional portfolios, such as utilities and financial services, will become available.

The management of Select Portfolios intends to keep each portfolio at least 80 percent invested in its specialized area. It is up to individual investors to decide which market sector they want to bet on. However, investors can switch their emphasis, and cash, from one portfolio to another, or into Fidelity Cash Reserves, a money market fund with assets of $2.8 billion.

"We don't advise our shareholders what to do," said Fidelity vice president Frank T. Parrish. "Everything that Fidelity does is done under the assumption that the reason people come to us is because they want to be their own managers. We are here to serve investors who can make their own decisions."

Parrish looks forward to a future in which aggressive financial service firms can develop under a more 'deregulated' atmosphere. The introduction of Fidelity Select Portfolios is, he said, an indicator of a new public awareness of the stock mar- ket and the possibilities therein.