The Securities and Exchange Commission, which has made an attack on insider trading one of its major missions in the past year, lacks adequate ammunition to win the fight, Commissioner Bevis Longstreth said yesterday.

In remarks prepared for delivery to the Federal Bar Association Securities Law Committee, Longstreth called on Congress to give the SEC tougher penalties to levy against those who profit from advance information about developments that affect stock prices.

Right now the commission can enjoin further violations and require violators to pay back the money they make. Cases also may be referred to the Justice Department for criminal prosecution, which carries a maximum penalty of five years in jail and a $10,000 fine. Criminal prosecution seldom occurs, he noted.

Paying back profits is "like using a pocket knife to fell a tree," said Longstreth. "How much deterrence can one expect from a sanction that simply compels the thief to return the stolen goods?"

Longstreth said that the commission should ask Congress to give the courts power to award damages to injured shareholders up to some fixed multiple--possibly 300 percent--of the insider's trading profits. Congress also should allow the SEC to seek a civil penalty in court of up to the same amount, he proposed.

He also said the SEC should ask Congress to permit the successful plaintiffs to recover costs of suing or to allow the corporation whose stock was traded to sue if investors do not.

Longstreth also said that the SEC might consider other remedies.