DeWitt S. Hyde, 77, who served 20 years as a municipal court judge in Washington, six years as a Republican congressman from Montgomery County and Western Maryland and six years in the Maryland General Assembly, died of pneumonia April 25 at Suburban Hospital.

As a member of the House of Representatives during the mid-1950s, Judge Hyde was vice chairman of the Joint Committee on Washington Metropolitan Problems. In that capacity he was instrumental in setting up extensive studies of transit, traffic and bridges and public financing problems in the area. He was a sponsor of legislation that created a Washington area transit regulatory commission, a step toward today's Washington Metropolitan Area Transit Authority.

A resident of Bethesda, Judge Hyde was appointed to a 10-year term on the old D.C. Court of General Sessions by President Eisenhower in 1959, less than a year after he was defeated in his bid for a fourth term in the House. He was named to a second 10-year term by President Nixon in 1969, and he retired in 1979 from what by then had become the D.C. Superior Court.

During his tenure on the bench he served as chairman of the Judicial Council Committee to study the operation of the Bail Reform Act in Washington. His reputation in the legal community was that of a reserved, hard-working and diligent jurist who rarely became involved in controversy.

A native Washingtonian, Judge Hyde was a graduate of the old Central High School and attended George Washington University and its law school. From 1933 to 1937 he was a legal correspondence clerk with the Farm Credit Administration, then opened a private law practice.

He served in the Navy in the South Pacific during World War II and for a year was the only officer in charge of 35 men stationed on the tiny island of Amadee, about the size of a city block, off the coast of New Caledonia. Their assignment was to watch for enemy submarines.

"All I had to do was sit and watch instruments and see that the men sat and watched instruments," Judge Hyde recalled. "It was terrifically boring."

But it was also during that period that Judge Hyde decided that a political career was one of the more worthwhile things a person could do. In Montgomery County after the war, he was part of a wave of veterans and new suburban dwellers ready to challenge the Democratic establishment. In 1946 he accepted the Republican nomination to the Maryland House of Delegates and was elected to the General Assembly. Four years later he was elected to the State Senate. In 1952 he won the first of his three terms in Congress.

At the time his congressional district included most of Western Maryland, and Judge Hyde was the first resident of Montgomery County to be elected to Congress in 53 years.

While Judge Hyde supported Eisenhower's general policies and programs in Congress, a large portion of his activity was related to local issues and problems. His committee also was among the first to call for an extensive study of pollution in the Potomac River, and the judge supported increased federal payments to the District of Columbia.

Judge Hyde was a founder of Christ Lutheran Church in Bethesda. He was a former chairman of Christ Child Church Center school for retarded children in Bethesda and a member of the Columbia Country Club, the Capitol Hill Club, the Veterans of Foreign Wars and the Bethesda Lions Club.

He is survived by his wife of 50 years, Mildred Hyde of Bethesda.