The process of valuing the securities in a mutual fund's portfolio - often an imprecise task - may require more work from now on for some fund companies and fund directors.

Funds must use fair value rather than market value to assign prices to the securities they own when financial markets do not open for an entire day and no other market prices are available, the SEC said in new guidelines it issued last week. And in the event that the market closing affects different companies listed on the same exchange differently - as in the case of an earthquake - funds and fund board members must try to value securities based on the particular circumstances of individual companies and not just the factors affecting the market as a whole, the SEC said.

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