Mr. Moss, a Democrat, served in the House of Representatives from 1953 until retiring at the end of 1978. He was guided throughout by the principle that Congress has a duty to help ordinary citizens when they cannot help themselves, whether in the marketplace or in dealings with public officials.

Although he never was chairman of a major committee, the usual prerequisite to power in the House, Mr. Moss was one of the most effective legislators of his day, and he left an indelible imprint in the field of product safety and government regulation. Consumer advocate Ralph Nader described him as "almost the prototypical congressman in terms of serving the people."

The source of Mr. Moss's power was his mastery of the art of congressional oversight, the exacting, unglamorous and frequently thankless process of ensuring that the various parts of the bureaucracy actually do the work for which they were created. For more than two decades, he chaired subcommittees that conducted hearings on a vast array of government and corporate misdeeds.

In 1977 and 1978, for example, he led inquiries on the following: the world uranium cartel, FBI foreign security surveillance, Air Force contract abuses, accounting practices, hospital care and unnecessary operations, natural gas shortages, drug costs, government policy on birth control, college athletics and the regulation of pesticides.

Earlier investigative targets included the use of flammable fabrics in clothing, lead paint and lead in Christmas tree ornaments, the safety of children's cribs and the need for childproof closures on packaging for medicines and poisons. 

The major legislative accomplishment of his career was passage of the Freedom of Information Act, an issue for which he struggled for more than a decade before it became law in 1966. Amendments adopted in 1975 strengthened it and broadened its effect.

Mr. Moss believed that citizens have a right to the information gathered with their tax money and that enforcing that right is essential to maintaining a democracy. But the idea of opening the government to scrutiny was anathema to the bureaucracy, to successive occupants of the White House and to important elements of both parties in Congress. Mr. Moss often found himself alone in advocating it.

Eventually the act was passed when Republicans bent on embarrassing the Democratic administration joined liberal Democrats in supporting it. A reluctant President Lyndon B. Johnson signed it into law.

Mr. Moss also was a principal force in establishing the Consumer Product Safety Commission in 1972 and the related reform of the Federal Trade Commission. The latter effort transformed the agency into an arm of consumer protection with the power to file class actions and seek court enforcement of its cease-and-desist orders.

In 1975, Mr. Moss sponsored amendments to the Securities Act that opened the industry to electronic marketing and ended such restrictive practices as fixed commission rates on stock transactions. Oversight hearings that he conducted contributed to the phasing out of the Federal Power Commission in 1977 and the assignment of its functions to the Energy Department and the Federal Energy Regulatory Commission.

Mr. Moss had a reputation for charting his own course. The reason was evident in his first term, when he voted against tidelands oil legislation, an issue of particular importance to California (and Texas). In the 1960s, his independence was manifest in his early opposition to U.S. involvement in the war in Vietnam. He went to Southeast Asia to see the situation firsthand, and he reported his findings to President Johnson in person.

In March 1973, when the Watergate scandal that forced President Richard M. Nixon to resign was still taking shape, Mr. Moss suggested that the House set up procedures for a bill of impeachment. He was the first congressman to raise the issue.

In 1975, he became one of the first congressmen to name a woman to the top staff position of administrative assistant when he gave the job to Kassy Benson, who had worked for him for many years.

Mr. Moss's work as a subcommittee chairman made him a hero to the large contingent of reform-minded new congressmen elected in the wake of the Watergate scandal. In 1977, they helped him unseat Rep. Harley Staggers (D-W.Va.) as chairman of the powerful Commerce subcommittee on oversight and investigation.

But he was never a member of the inner circle of the House of Representatives. Colleagues regarded him as too much of a maverick. Although his staff almost worshiped him as a father figure, his public image was that of a relentless and sometimes abrasive investigator. In addition, he had a reputation for saying what was on his mind without regard for the niceties of congressional protocol. Critics claimed variously that he was a publicity monger, a left-winger or an enemy of business.

None of that fazed him. In an interview at the time of his retirement, he declared that "too many people want to be popular around here. I don't really give a damn. If it's the right vote, it will become popular."

To the idea that he was anti-business, he replied: "Actually, I am very conservative. People who want to dismantle the regulatory machinery of government are not conservatives, as many of them claim. They are very radical. That machinery is there because abuses occurred: the FTC, the Federal Reserve, the Securities and Exchange Commission and the Interstate Commerce Commission. These things you just don't tear down and destroy. Those cries come from people who claim regulation is onerous."

John Emerson Moss Jr. (he dropped the Jr.) was born April 13, 1915, in Hiawatha, Utah, where his father was a miner. When he was 10, his family moved to Sacramento. In 1933, he completed a two-year course at Sacramento Junior College. He had different jobs until 1938, when he became the owner of an appliance store. During World War II, he served in the Navy. When he returned home, he went into the real estate business with his brother.

Mr. Moss's career in politics began early. In 1938, he was named director of the California Young Democrats and a member of the California Democratic State Central Committee -- he held the latter position until 1980. In 1942, he was chosen Young Democratic National Committee member from California.

In 1949, Mr. Moss was elected to the California State Assembly, where he was an assistant Democratic leader. He remained there until 1952, when he was elected to Congress from the newly formed 3rd District, which included Sacramento and part of its surroundings. He was reelected 12 times by comfortable margins.

In 1978, he chose not to run again, in part because of injuries he received in a car accident and in part because he wished to return to California and spend more time with his family. He headed a bank in Sacramento and retired completely in the 1980s. He moved to San Francisco for reasons of health.

Survivors include his wife, the former Jean Kueny, whom he married in 1935, of San Francisco; two children, Jennifer Moss of San Francisco and Allison Moss of Sacramento; and four grandchildren.