Former Congressman John Jarman, 66, who served 26 years in the House of Representatives, died Friday at Presbyterian Hospital here. He had micosis fungoides, a rare form of skin cancer.

Mr. Jarman had retired in early 1977, at the end of his 13th term. While always elected as a Democrat from Oklahoma, he switched to the Republican Party midway in his final term.

In switching party registration in 1975, Mr. Jarman said Democratic reformers in the House were in the process of "nullifying the seniority system and punishing those who do not adhere to the liberal party line."

Mr. Jarman captured the congressional seat in 1951. While a Democrat, he became chairman of the subcommittee on Interstate and Foreign Commerce and served on the Foreign Affairs, Post Office and Civil Service committees.

Republicans assigned Mr. Jarman to the Science and Technology Committee. Jarman's switch surprised few people. He often had voted along Republican Party lines.

Mr. Jarman was born in Sallisaw, Okla. After service in the Army in World War II, he was elected to the Oklahoma House of Representatives in 1948. He was elected to the state Senate in 1948 and to Congress from the 5th District in 1950.

He received a bachelor's degree from Yale University in 1937 and a law degree from Harvard in 1941. He practiced law with his father, J.H. Jarman, in Oklahoma City until entering the military.

His first wife, Ruth, died in September 1964. He married the former Marylin Grant in 1968. His second marriage ended in divorce. Survivors include three children by his first marriage, and five grandchildren.