Investors who want to make money in mutual funds have generally faced a pair of mysteries: the market itself, and their fund's fee structure.

Now, though the market remains as inscrutable as ever, the government is moving to lift some of the clouds surrounding the charges that funds levy.

Late last month the Securities and Exchange Commission voted to require mutual funds to lay out their fees in tabular form in their prospectuses and to compute what these fees would add up to on a $1,000 investment over several time periods.

The agency also adopted new rules to standardize the way mutual funds compute the performance figures used in their advertising.

The intent of the fee rules, according to Kathryn McGrath, director of the SEC's Division of Investment Management, is to make it possible for investors to determine what a fund's fees add up to and to do so in a way that allows them to compare funds.

As it is now, the fees are included in fund prospectuses, but they are scattered through the text so that "it's very hard for the investor to tote it all up," McGrath said. And with the addition of new kinds of fees over the past five years, she said, adding them all up in a way that allows comparison of funds "is as a practical matter just not doable."

Different funds charge different fees, or combinations of them. While it was once possible to categorize funds as "load," "low-load" and "no-load" based on their upfront charges, today this vastly oversimplifies the situation.

All funds charge management fees, typically about 0.5 of the net asset value of the fund annually. In the case of a no-load fund, marketed directly to investors by the operator, this may be the only fee, but such "pure" no-load funds are getting harder to find.

Load funds, marketed through brokers and other sales representatives, charge a sales fee -- the load -- that is used to pay commissions to the sales people and to cover other costs associated with distributing the fund's shares. These loads can be as much as 8 1/2 percent of the value of the shares.

Increasingly, even funds that are sold directly are adding an upfront charge to help cover the services they are providing, such as 24-hour toll-free telephone service. These have become known as low-load funds.

Investors can determine a certain amount about a fund's fees by looking at the list of mutual fund prices that appears amid the stock market tables each day in this and other newspapers.

The funds are grouped by operator, and each fund is followed by three numbers. The first is the net asset value of the fund per share. The second is the asked, or offer, price, which is the price an investor would have to pay for a share; the difference between the two is the load (in a no-load fund the second number is replaced by the letters NL). The third number is the change in value from the previous day.

Some funds also impose loads on dividends that the shareholder opts to have reinvested. Under the new SEC rules, funds that charge such loads on reinvested dividends would have to include them in the fee table.

For many years, management fees and loads constituted the bulk of mutual fund fees. But in recent years, other, harder-to-spot fees have increased dramatically in popularity among fund operators.

Foremost among these are 12b-1 fees, so-called after the section of the SEC rules that governs them. Under Section 12b-1, fund operators may use some of the fund's assets for promotion, marketing and distribution. These charges can amount to more than 1 percent of the fund's assets annually.

More and more funds are charging redemption fees or deferred sales charges. These "back-end loads," assessed when the investor sells his or her shares, often run to 5 percent of the value of the shares. In fact, a fund that appears in newspaper listings as a "no-load" can in fact be charging investors very substantial fees.

Under the new rules, which take effect for most funds on May 1, a fund's prospectus will have to include a description of its fees and a table listing them. The table will have to show loads on share purchases as a percentage of the offering price; loads on dividend reinvestments, also as a percentage of the offering price; deferred sales loads, as a percentage of the original purchase price or of the redemption proceeds, whichever is applicable; redemption fees, as a percentage of the amount redeemed; and exchange fees, which are sometimes charged if an investor switches his account to another fund.

The table must also list annual charges that are taken from the fund's assets -- management fees, 12b-1 fees and other expenses, as a percentage of the fund's average net assets.

Finally, there must be an example of the expenses an investor would pay on a $1,000 investment -- assuming a 5 percent annual return -- if it were redeemed after one, three, five and 10 years. It must also show what those expenses would be if the investment were not redeemed.

In a parallel development, newspaper readers will also be getting more data soon. The National Association of Securities Dealers, which collects the mutual fund quotations that appear in financial sections, plans to add a footnote, designated by the letter "p," to the tables to indicate which funds charge 12b-1 fees. The tables already include an "r" footnote to designate those charging redemption fees.

The SEC's new advertising rules will compel fund operators to use a standard formula when computing such numbers as annual yield, tax equivalent yield and total return. The requirements, effective May 1, will apply to both advertising and sales literature.

These requirements are aimed mainly at bond funds, though they cover other types, as well. They grew out of a controversy that arose some three years ago when interest rates, declining from historic highs, pushed bond fund returns to very high levels. The SEC and industry groups grew concerned that some investors would be misled.

In addition, there are numerous ways to compute yields, so that investors reading ads could not be sure that a fund advertising a higher yield was really a better investment.