Investors spooked by stocks and most bonds have found relief in money-market mutual funds — but that comfort may be deceiving.

Despite recent improvements in the credit markets, many money-market funds are providing positive yields only because of waivers on management fees. In other words, fund firms are taking a hit so their shareholders don’t.

Starting late last year, many firms waived some fees, and even closed certain funds to new investors, as interest rates fell in response to the freeze in the credit markets. Six months later, money-fund investors are seeing negligible returns while firms are forgoing revenue in difficult times.

Fee waivers ensure individual or “retail” investors aren’t losing money and in fact may enjoy a few percentage points of extra return. And as long as the Federal Reserve’s target overnight lending rate among banks is between zero and 0.25%, fund providers are essentially stuck.

“What can [the funds] do?” asked Connie Bugbee, managing editor of iMoneyNet, which tracks money-market offerings.

Perhaps 90% of money-market funds have fee waivers now, up from the typical level of about 60%, Ms. Bugbee said.

Major firms that have waived fees on some of their money-market funds include DWS, a unit of Deutsche Bank AG, Fidelity Investments, Federated Investors Inc. and BlackRock Inc.

“We have been waiving fees for money-market funds where the gross expense ratio is greater than total gross performance,” said Mayura Hooper, spokeswoman at DWS.

“As is consistent with much of the rest of the industry in the current low-rate environment, we are waiving fees or reimbursing expenses on some of our Treasury-only and government money-market funds,” to ensure there aren’t negative yields, added Alexi Maravel, a Fidelity spokesman.

Retail investors may have taken notice that money funds aren’t the bargain they once were. Through May 19, retail money-market assets this year have fallen 6%, to $1.28 trillion, according to iMoneyNet. In 2008, retail assets grew by 9.1%.

Given the tight rules over what money-market funds can own, fund firms don’t have much room to find higher-yielding securities that could end fee waivers. And in a risk-averse market, taking such steps mightn’t be what investors want. So there is little fund companies can do except wait for rates to rise.

Yet, particularly for smaller firms, waiving fees indefinitely — or until the Fed raises rates — mightn’t be feasible.

“These funds don’t have the economies of scale” to absorb fee waivers that larger fund firms enjoy, Ms. Bugbee said.

Accordingly, she noted, many firms may look at their money-market business and ask whether it is worth the trouble. “Some of these funds were created as a convenience” for retail investors waiting to put their cash into stock or bond funds, Ms. Bugbee said, rather than as a core part of a firm’s business.