BOSTON -- Three words investors in money-market mutual funds never want to hear: "Break the buck." It refers to an event that's happened just twice in almost 40 years. Financial markets tanked, and managers weren't able to keep fund shares at the steady $1 price they're supposed to maintain to protect investors from losses.

It's not a guarantee. But there's an expectation that draws safety-minded investors to money funds: Invest a buck, get at least a buck back, on demand. The chief goals are safety and on-demand access to cash, rather than strong returns. The stable $1-a-share feature distinguishes money funds from other mutual funds whose share prices veer with the stock and bond markets.

Yet it's not unusual for money funds to fluctuate slightly above or below that $1 safety threshold, based on how the market values a fund's holdings from day to day. Fund managers have leeway so that the price at which investors buy or sell shares stays at $1, even when the current market value for the funds' holdings suggests a price of 99 and nine-tenths of a cent would be more accurate.

This happens mostly behind the scenes, and involves what the funds call their shadow prices. But starting Monday, this information will become public. More than two years after a money fund's collapse prompted new safety rules, the Securities and Exchange Commission will begin publishing shadow prices.

The industry and investor advocates alike say disclosures about shadow prices dipping below a dollar shouldn't be a cause for alarm.

"You are going to have some very risk-averse investors who will be uncomfortable about finding this out," says Niels Holch, executive director of the Coalition of Mutual Fund Investors. "But if investors are properly educated, I don't think there will be a mass of people rushing to sell. I think it's a matter of the industry telling individual investors how this works.

The disclosure requirement comes as the industry continues damage-control after the Reserve Primary Fund debacle. That fund's shares fell to 97 cents after its managers disclosed that a $785 million bond investment in Lehman Brothers had become worthless when the investment bank went bankrupt. Fearful clients, mostly institutional investors, demanded cash back, and managers were forced to sell holdings at steep discounts as the market plunged.

Although nearly all shareholder cash has since been returned, it was the first time individual investors got stuck in a money fund that broke the buck. A fund catering to institutional clients broke the buck in 1994, the first time that happened since money funds came into existence in 1971.

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