Prosecutors in the Wayne B. Williams murder trial today tried to show that fibers in Williams' green bedroom carpet, which they believe link him to the two young men he is charged with killing, are unusual and were manufactured only in small quantities.

The fibers in Williams' carpet are crucial to the prosecution's case. Experts from the Georgia and FBI crime labs are expected to testify that those fibers are similar to fibers recovered from the bodies of Nathaniel Cater, 27, and Jimmy Ray Payne, 21. Williams is accused of killing Cater and Payne, who are among the last of 28 young Atlanta blacks whose deaths over a 22-month period are being investigated by a special police task force and the FBI.

To lay the groundwork for that testimony, prosecutors today asked one expert from the E.I. du Pont de Nemours & Co. to lecture the jury on the synthetic textile fiber industry.

With the aid of four-color posters, an overhead projector and a blackboard, the expert, Herbert Pratt of Du Pont's textile fibers department, explained that synthetic fibers are "engineered" for specific uses and can be differentiated by their color, shape, size and chemical composition.

Pratt also testified that the FBI asked him to help them identify the manufacturer of a specific kind of fiber, apparently taken from Williams' carpet.

In his 30 years of experience in the fiber industry, Pratt said he had never seen any fibers like the ones the FBI showed him.

It was not until several days later, while Pratt was having lunch with a woman technician who photographs fibers through microscopes, that anybody at Du Pont recognized the fiber.

Pratt said he sketched the fiber's cross-section on a napkin, and the technician told him it was similar to a fiber photograph she had saved in her desk because it was so unusual.

The fiber was produced by the Boston-based Wellman Inc., and made into carpet by the West Point Pepperell Co. of Dalton, Ga.

Henry Poston, Wellman's technical services manager, then testified that the company manufactured the nylon fiber in question from 1967 to 1974. He said the fiber's unusual cross-section was the result of his firm's efforts to "circumvent" the patent laws protecting a Du Pont design it was modeled after.

Finally, the prosecution brought in Gene Baggett, the purchasing manager of West Point Pepperell's carpet and rug division, to testify that his firm bought raw fibers from Wellman only during the calendar year 1971.

Baggett identified a carpet sample that had been provided to both the FBI and the defense team as "Luxair" carpet in the color "English olive" that had been manufactured by his company.

Baggett also testified that West Point Pepperell sold about 54,500 square yards of Luxair carpet in 16 colors during 1971, and that about 9,000 square yards were sold in a 10-state region that includes Georgia during the first six months of 1971.

The prosecution has not yet introduced the carpet that police seized from Williams' home after his arrest in late June, but it is clear that prosecutors are trying to establish that carpets like Williams' are rare in the Atlanta area, and that the similar fibers found on the victims' bodies came from his house.

The defense, which will begin its cross-examination of Baggett tomorrow, is trying to establish that the fibers found on the victims' bodies could have come from a source other than Williams.