Two of the biggest names in the mutual fund industry have created new money market funds boasting yields that beat all their older competitors.

They are getting an enthusiastic response from savers and investors scrambling after the high returns available in short-term money market investments.But experts on mutual fund investing urge that you look beyond the current yield figures these funds sport before you decide whether they suit your purposes.

In figures compiled early this month by Donoghue's Moneyletter, the Dreyfus Worldwide Dollar Money Market Fund and the Fidelity Spartan Money Market Fund ran neck and neck for the No. 1 spot with seven-day yields of 10.4 percent.

The average yield of 542 taxable money funds, by contrast, was 9.08 percent.

How can Dreyfus and Fidelity do this? Well, for one thing, their timing was fortuitous. Since the new funds were organized in a period when interest rates were climbing, they were able to stock their portfolios with the latest high-yielding securities the marketplace had to offer.

Older funds, by contrast, could accumulate new, higher-yielding securities only gradually as their assets increased and existing investments in their portfolios matured.

More to the point, however, is a marketing strategem that has been used many times before when a new fund is introduced: Dreyfus and Fidelity are temporarily paying the funds' expenses and waiving their management fees.

"It's smart to take advantage of these situations, but watch these funds closely," says Sheldon Jacobs in his newsletter The No-Load Fund Investor.

"They can stop waiving expenses without notice and their yield advantages will disappear."

The charges Dreyfus isn't collecting effectively increase the Worldwide Dollar fund's yield by 0.7 of a percentage point, notes Norman Fosback, editor of the newsletter Income & Safety, while the difference at Fidelity Spartan is 0.6 of a point.

"Frankly, we are ambivalent regarding this promotional technique," Fosback says.

"The upside is that such funds offer adroit investors an opportunity - albeit brief - to earn far-above-average returns. The downside is that savers seeking these opportunities may end up with their money in a fund whose performance may prove to be below average in the long run."

Even after they have stopped waiving fees and expenses, it's possible that the Worldwide Dollar and Fidelity Spartan funds will do well.

Fosback says the Worldwide Dollar fund, with a somewhat different investment policy, can be expected to outperform by at least a small margin its huge sister fund, Dreyfus Liquid Assets.

Fidelity, meanwhile, says the Spartan Fund will continue to carry lower fees and expenses than the long-established Fidelity Cash Reserves fund, in part because Spartan shareholders will pay separate charges for services offered "free" to investors in most other money funds.

As Donoghue's Moneyletter advises, "Make sure your fund is a consistent performer."