Giulio Natta, 76, a pioneer in plastics research and cowinner of the 1963 Nobel prize in chemistry, died Wednesday in a Bergamo, Italy, hospital. He had Parkinson's disease and had undergone surgery for a broken thigh.

He won the Nobel prize with Karl Ziegler of West Germany for contributions to macromolecular chemistry and the development of high polymers.

In making the award, the Nobel academy said the two chemists had helped develop basic chemical techniques and their work had led to "a number of new products in the fields of plastics, such as laundry detergents and antiknock admixtures to high octane fuels for engines."

Dr. Natta began his research in polymers, the molecules that form organic solids such as proteins and rubbers, in the 1930s.

During this time Italian dictator Benito Mussolini was pushing Italy toward "self-sufficiency" following League of Nations sanctions against Italy for the 1935 invasion of Ethiopia.

Dr. Natta later termed this "an element of luck" in his scientific career because "it forced chemists to venture into the field of new matter."

His early work in polymers involved synthetic rubber. He developed syntheses for methanol and formaldehyde during the war.

In the late 1940s, he began working with olefins, a natural gas derivative that was susceptible to polymerization, the uniting of hydrocarbons necessary for changing the gaseous state to a solid plastic one.

Dr. Natta and his team concentrated their research on propylene, an olefin thrown off in the refining of oil and the processing of propane.

During this period, Karl Ziegler was concentrating his research on ethylene, the most promising of the olefins from the commercial point of view. He used a catalyst based on titanium trichloride and aluminum to produce the plastic resin polyethylene.

Dr. Natta's research took him to Frankfurt, Germany, where he attended a lecture given by Ziegler."Something of what I heard got under my skin and to my brain," Dr. Natta recalled. Later, he invited Ziegler to Milan to view his own findings.

By 1954, using the catalytic principle of Ziegler, Dr. Natta and his team isolated isotactic polypropylene, a substance 10 times cheaper than ethylene, and had it on the market two years later.

Dr. Natta demonstrated that the new plastic was stronger and more heat resistant than other plastics. It could be spun into fibers lighter and as strong as nylon or spread into a film as clear as cellophane.

During the 1950s, he developed several other polymers, including a new polystyrene and a polybutadiene.

His strenght, according to the Nobel academy, was in bridging the gap between the natural and the synthetic, and in linking simple atoms into complex molecular networks.

Dr. Natta was born in Imperia, on the Italian Riviera, and reared in Genoa. He earned a doctoral degree in chemical engineering at the Polytechnic Institute of Milan in 1924. He taught at the universities of Pavia, Ome and Turin and then returned to the Polytechnic Institute in 1938 as director of the Institute of Industrial Chemistry.

He was a member of the National Academy of Sciences of Italy, the Rotary Club of Milan and an honorary member of the New York Academy of Sciences.

Survivors include a son and a daughter. CAPTION: Picture, GIULIO NATTA