A story in the July 29 edition incorrectly reported that the Supreme Court had ruled that postal regulations authorizing U.S. customs agents to open mail suspected of containing contraband were not constitutional. The court ruled that such regulations are constitutional.

Evidence that U.S. customs agents have illegally opened and read foreign and domestic mail for at least the past six years was disclosed yesterday during a hearing by a House Government Operations subcommittee.

C. Neil Benson, chief inspector for the Postal Service, acknowledged that such abuses have occurred. In a letter and in testimony before the Government Information and Individual Rights Subcommittee, he revealed that.

Customs agents in New York, as a matter of "general policy," seized and turned over to the military for investigation narcotics sent to or from military personnel and personal correspondence that accompanied it. This was done without a search warrant.

In July, 1975, postal officials in Port Isabel, Tex., permitted customs agents there to open domestic first-class mail in violation of the law.

In September, 19th, a customs agent in Hixson, Tenn., not only read three personal letters from abroad to a citizen who was the subject of a criminal investigation, but also hired someone to translate the letters from Chinese.

Customs sometimes detains mail for up to 90 days while it consults other investigative agencies.

Benson said such practices were wrong, but blamed them on the Customs Service, not the Postal Service. In 1971 the agencies signed an agreement allowing customs agents to open sealed mail from abroad if there was "reasonable cause" that it contained contraband or dutiable goods.

Last month the Supreme Court ruled 6 to 3 that postal regulations authorizing customs officials to open - but not read - mail suspected of containing contraband were not constitutional. But subcommitee members appeared angry yesterday that the Postal Service was not exercising the oversight it should over the Customs Service.

"There seems to be a policy here," said Rep. Leo J. Ryan (D-Calif.), "that whatever customs wants it gets - even if its policy is different than yours. I'm incensed. I want some resistance by the Postal Service to this reprehensible policy."