Arthur Andersen & Co., one of the Big Eight accounting firms, has been found guilty of defrauding the shareholders of a mutual fund more than a decade ago and ordered to pay $80.7 million to those investors.

The huge civil verdict, returned by a six-member jury Tuesday in U.S. District Court here after 13 days of deliberations, will be challenged by the accounting firm, according to its attorney. He said the firm will attempt to vacate the "excessive" award and failing that will seek a new trial.

The case dated to 1968, 1969 and 1970, when Andersen was auditor for the mutual fund called Fund of Funds. The off-shore fund, then worth about $750 million, was part of the financial empire created by Bernard Cornfeld and operated out of Europe.

Fund of Funds was put into liquidation by the Supreme Court of Ontario in 1973, and the court-appointed liquidator brought suit against Andersen. The liquidator is John Orr, a partner in the Toronto office of Touche Ross, another major accounting firm.

During the trial, which began July 7, it was alleged that the accounting firm had been aware that another of its clients, John M. King, chairman of King Resources in Denver, had defrauded Fund of Funds. King was convicted and went to prison for criminal fraud as a result of these same financial dealings with Fund of Funds.

The civil suit against Andersen was brought on behalf of three related groups: Fund of Funds Ltd., F.O.F. Proprietary Funds Ltd. and IOS Growth Fund Ltd.

During the trial, the plaintiffs alleged that at a Fund of Funds board meeting in 1968, King proposed that the fund invest assets in natural resources run by King Resources.

But it turned out that King grossly inflated the price of the investment property. For example, King allegedly bought a lease-hold interest in arctic property for $18,000, then sold it to Fund of Funds for $1.9 million. In another case, the jury was told that King acquired an interest in a South African diamond mine, paying only administrative costs of $11,000. He sold it to Fund of Funds for $6 million.

The other major claim in the case was that King sold a portion of his property in the arctic for a huge amount, causing the value of the arctic holding he sold to Fund of Funds to be valued upward by the fund's management. But it turned out that the King deal was not at arm's length and the figures were fraudulent.

Arthur Andersen, as auditors for both King and Fund of Funds, participated in the revaluation and price-inflation scheme, the plaintiffs alledged.