Seven members of the billionaire Hunt family of Texas yesterday agreed jointly to pay a $500,000 fine for violating the rules of the soybean market in 1977.

The fine is the largest ever imposed by the Commodity Futures Trading Commission, but is a small fraction of the profits the Hunts are believed to have made in their soybean-buying spree.

The CFTC charged that brothers Nelson Bunker Hunt and W. Herbert Hunt and five of their children bought contracts for almost 24 million bushels of beans -- eight times the 3 million bushel limit set by the government.

Accusing the Hunts of more than 300 separate violations of the soybean limit, federal commodity regulators took the Hunts to court in April 1977, beginning a 51-month legal fight that ended yesterday with a negotiated settlement.

As is usual in such cases, the Hunts neither admitted nor denied they did anything wrong, but agreed to accept the penalty imposed.

"As every businessman or corporate executive knows, there comes a time after years of expensive and protracted controversy to reach a settlement," said family spokesman Tom Whitaker.

Originally, the government tried to force the Hunts to pay back the profits they earned when the price of soybeans jumped from $6.90 a bushel to $10.35.

At the time, the Hunts were estimated to have made at least $30 million and perhaps as much as $100 million on their 24 million bushels of beans.

But the CFTC gave up trying to make the Hunts give back their profits and wound up settling for the record fine and a court injunction.

"Pursuing that remedy was not a good use of our resources," said Thomas Loughran, director of CFTC's enforcement division. Loughran insisted that the fine and court order were the toughest penalty the agency could have expected to get.

The Hunts' ability to muster top-flight legal talent to fend off government charges is well-known. Last week, Hunt lawyers obtained a federal injunction restricting a Securities and Exchange Commission investigation of their silver trading last year. The CFTC also is investigating whether the Hunts were manipulating the market when silver prices jumped from less than $10 an ounce to $50 and then collapsed, costing the Hunts more than a billion dollars.

The $500,000 fine imposed by the CFTC is less than the interest the Hunts pay every day on the $1.2 billion loan they took out to cover their silver losses.

Besides the fine, the Hunts also agreed to abide by a permanent injunction prohibiting them from exceeding the limit on holdings of any commodity and prohibiting them from using the soybean futures market for two years.

Loughran said yesterday's settlement will not affect several private lawsuits filed against the Hunts by other soybean traders who lost millions of dollars when the price of beans jumped almost $3.50 a bushel.

The CFTC did not accuse the Hunts of manipulating the soybean market, he stressed, only of exceeding the limit on the number of soybean futures contracts that any speculator can own.

The government sets limits on the amount of commodities that a speculator can control in order to prevent someone from buying so much that they could corner the market and drive up prices.

The CFTC took the Hunts to court after finding that seven members of the family, working through various brokers, had bought contracts for nearly 24 million bushels of beans, when the limit on holdings was 3 million bushels.

The Hunts claimed they did not violate the limit because each of them individually owned less than 3 million bushels. They insisted their purchases were not made as a group.

But CFTC officials charged the Hunts were acting in concert, and federal courts upheld that charge. The CFTC complaint said Bunker and Herbert Hunt opened soybean trading accounts for their children and shifted beans into the youngster's names to circumvent the limit.

Besides Bunker and Herbert, the best-known children of the late oil magnate H. L. Hunt, the soybean case involved Herbert's son Douglas and Bunker's children, Houston Bunker Hunt, Ellen Hunt Flowers, Mary Hunt Huddleston and Elizabeth Bunker Hunt.