Mstislav V. Keldysh, a leading Soviet scientist and mathematical who was prominent as a spokesman for the Soviet space program while head of the prestigious Soviet Academy of Sciences, died Saturday at the age of 67.

The Soviet news agency Tass described the death as sudden, but gave few other details.

Five years ago Mr. Keldysh underwent 6 1/2-hour operation on his circulatory system. Dr. Michael DeBakey, a renowned American surgeon, traveled to the Soviet Union to perform the surgery.

Recognized for his abilities in a wide range of scientific and mathematical disciplines bearing on aviation and rocketry, Mr. Keldysh also approved himself an able administrator possessed of the political acumen necessary to rise to the top of his country's scientific establishment.

As president of the Academy of Sciences from 1961 to 1975, Mr. Keldysh held a post that made him not only the spokesman for Soviet science, but also a key figure in the development and organization of scientific research throughout the Soviet Union.

Early in his tenure as chief of Soviet science, Mr. Keldysh delivered a sharp rebuke to the school of biology favored by Nikita Khrushchev and represented by Trofim Lysenko.

Briefly stated, it was the view of that school that environmental influences could bring about changes in plants and animals that could be passed on to future generations. Western scientists had long rejected the view as unfounded, but some Soviet leaders found it appealing from a political standpoint.

In firmly rejecting it as impeding scientific progress, Mr. Keldysh was credited in the West with helping to restore the science of genetics, one of the most active area of modern biology, to the Soviet Union.

Born in Riga, Latvia, Mr. Keldysh was the son of a civil engineer and a member of an intellectually prominent family. Shortly after graduating in mathematics and physics from Moscow State University, he joined the Central Aero-Hydrodynamics Research Institute, then known as the major Soviet center for aircraft development.

While carrying out research in aero-dynamics and fluid mechanics, and the theory of oscillations, waves and vibrations, he continued his studies and obtained a doctorate.

His work had important applications to the development of high-speed and long-range rockets and jet aircraft, and for the solution of such particular problems as the launching of missiles from submarines.

He won a Stalin Prize in 1942 for work published under the title, "The Theory of Calculating and Developing methods of Reducing Various Types of Vibrations in Aircraft."

He received another such prize in 1946 for a 1945 monograph, "The Front-Wheel Shimmy of the Tricycle (Aircraft) Landing Gear." The three-wheel landing gear, using a nose wheel rather than one under the tail, has been in general use since the 1940s.

In 1943, Mr. Keldysh became head of a top secret aircraft development institute. In succeeding years, as his own works multiplied and he branched into mathematical computing techniques, he also assumed increasing administrative responsibilities.