James B. Fisk, former chairman and president of Bell Laboratories and a key figure in the development of radar, died Monday at the Elizabethtown (N.Y.) Hospital. He was 70 years old and lived in Basking Ridge, N.J.

Mr. Fisk did atomic physics research and during World War II headed a group that developed the microwave device used to generate highfrequency radar. In that project, Mr. Fisk showed he could effectively manage other scientists, including theoreticians unused to working on concrete problems.

That skill fueled Mr. Fisk's rise as an executive at Bell Labs. The labs, with headquarters in Holmdel, N.J., are the research and development arm of the American Telephone and Telegraph Company and one of the world's leading centers for basic science research.

Mr. Fisk became the president of Bell Labs in 1959 and held that post for 14 years. He became chairman of the board in 1973, a year before his retirement. Assembled Research Teams

Bell executives credit Mr. Fisk and his predecessor as president, Mervin Kelly, with anticipating the electronics revolution that was built on technological advances during World War II. Together, they assembled research teams that were responsible for such inventions as the transistor.

During Mr. Fisk's tenure as president, Bell Labs produced a steady stream of electronics inventions, innovative lasers and satellite communications systems.

Throughout his administrative career, associates said, Mr. Fisk accepted the nonconforming behavior and ideas that often characterize innovative scientists and leavened his performance with an impish sense of humor.

''He used to dress impeccably but would occasionally preside at conferences wearing an engineer's hat,'' recalled Sidney Millman, a retired Bell colleague. Served in A.E.C. Post

Mr. Fisk left Bell on several occasions for government service. In 1947, he served as the first director of research for the United States Atomic Energy Commission. In 1958, he headed a team of scientists sent to Geneva by President Dwight D. Eisenhower to negotiate the technical groundwork for the nuclear test ban treaty.

The Geneva assignment represented a rare brush with the limelight for Mr. Fisk. Mr. Fisk subsequently established a reputation as a tough negotiator who won the respect of his Soviet counterparts. He later served Presidents John F. Kennedy and Lyndon B. Johnson on various committees.

In 1972, he was a member of the American delegation sent to Europe to discuss implementation of the Moscow summit agreement on cooperation in science and technology. Taught Physics at M.I.T.

Before joining Bell in 1939, Mr. Fisk taught physics at the University of North Carolina and the Massachusetts Institute of Technology. He held a similar position at Harvard University in 1948, while on leave from Bell.

He maintained close ties to the academic world throughout his career, including serving at various times on the governing boards of M.I.T., Harvard and the New Jersey Institute of Technology.

He was a member of many scientific and professional societies and was a trustee of both the John Simon Guggenheim Memorial Foundation and the Alfred P. Sloan Foundation. He was a director of the American Cyanamid Company, the Equitable Life Assurance Society, the Cummins Engine Company, the Corning Glass Company, the Cabot Corporation and the American National Bank and Trust Company in Morristown, N.J.

Mr. Fisk was born on Aug. 30, 1910, in West Warwick, R.I. He received a Bachelor of Science degree and a doctorate in theoretical physics from the Massachusetts Institute of Technology. From 1932 to 1934, he studied at Cambridge University under a Proctor Traveling Fellowship.

He was appointed assistant director of research at Bell Labs in 1949. He became vice president for research in 1954 and executive vice president the following year.

Mr. Fisk is survived by his wife, Cynthia, and three sons, Samuel of Concord, Mass., Charles of Cambridge, Mass., and Zachary of La Jolla, Calif., and four grandchildren.

A funeral service will be held Saturday at 4 P.M. in the Keene Valley (N.Y.) Congregational Church. A memorial service will be held at a later date in New Jersey.

Illustrations: Photo of James B. Fisk