A key prosecution witness testified today that she saw accused killer Wayne B. Williams sitting in a park with Nathaniel Cater less than two weeks before Cater's nude body was recovered from the Chattahoochee River.

Margaret Carter, the first witness to place the freelance photographer and self-styled talent scout with one of his two alleged victims, said she saw Williams, 23, sitting on a picnic table beside Cater nine days before his body was found in the river May 24.

"Are you sure he was the same [man] you saw sitting on the bench with Nathaniel Cater?" asked Jack Mallard, an assistant county prosecutor.

"Yes, I am," Carter said in a firm voice. What struck her about Williams were his clothes, she said: a silver silk shirt over gray trousers. "I remember him because he was well-dressed. I ain't ever seen a well-dressed man just sitting in the park like that."

Williams is accused of murdering Cater, 27, a day laborer, and Jimmy Ray Payne, 21, an ex-convict, and dumping their bodies in the Chattahoochee River. The two men are counted among a list of 28 young blacks killed or missing here in the period between July, 1979, and last May.

Carter said she recognized Williams from television accounts as the man she had seen with Cater, a friend who once dated her niece, Jeanette.

"Where's Slim?" she said Cater asked her. "Slim" was Jeanette's nickname.

Carter, in her 40s, testified that she saw a white Chevrolet station wagon in the parking lot nearby. A German shepherd dog was bounding about the park, too, she said.

Williams owns a German shepherd named Sheba, and drives a beat-up, beige Chevrolet station wagon. Prosecutors are trying to prove that he used that car to drive Cater's body to the Jackson Parkway Bridge early on May 22.

She stuck by her account under grueling cross-examination by defense attorney Alvin Binder. She budged only once, changing the car's color to blue. She called Cater a friend.

"Did you know Nathaniel Cater took a drink now and then?" Binder asked.

"Yes," Carter said.

"I bet you took a drink with him, didn't you?"

"I sure did."

Two witnesses--the manager of the Falcon Hotel, where Cater lived in a $4-a-night room, and a nurse who frequently drew blood from him at a downtown plasma center--testified today that they saw Cater May 21.

That was the day before a police sentry heard a splash in a river and Williams' car was spotted driving slowly across the bridge above. The testimony countered Binder's attempt to suggest Cater had been in the water longer than two to five days, as Dr. Robert Stivers, the Fulton County medical examiner, had testified earlier today.

While there was no external evidence of strangulation, Stivers suggested that Cater could have been killed with a forearm choke hold, which he said leaves few marks.

He described the slightly bruised larynx and the elevated blood alcohol in Cater, a quiet drunk who, on the last day anyone saw him alive, was rejected by the blood plasma center because he had too much protein in his blood.