Arthur J. Walker is hardly an imposing figure as he sits hunched forward at the defense table in the massive granite, Roman-columned Courtroom 301 in the federal courthouse here. A tiny, balding man, his glasses are too big, and even former co-workers testifying against him sometimes don't recognize him because he's not wearing his brown hairpiece.

Arthur Walker, standing trial on espionage charges as the first defendant in the alleged Walker spy ring, is hardly a master spy, either.

His lawyers say Walker, a 50-year-old retired Navy lieutenant commander, was manipulated during a period of near bankruptcy by his brother, John, the alleged spy ringleader, into passing confidential documents. But John mostly chided Arthur because the documents were so low level and uninformative, court documents show.

Then, once his brother was arrested, Arthur Walker was easily manipulated by the FBI, in 35 hours of interviews over four days, into volunteering almost everything he knew about the alleged spy ring -- all without a lawyer. He gave them more after he was formally arrested.

The government has built much of its case against Arthur Walker -- and a large part of its case against brother John -- on what Arthur blithely told FBI agents, according to court testimony in the last two days, a reading of court documents and interviews.

"He's a victim," said Arthur Walker's lawyer, Samuel Meekins. "Not a total victim, because Arthur has done some things that are wrong, and he's willing to pay for this . . . but he's incredibly naive. He's the most innocuous criminal of all time."

Charged in the alleged Soviet spying case are Arthur Walker; John Walker, 48, a private investigator and retired Navy chief warrant officer; John's son Michael, 22, a Navy seaman, and John's friend, Jerry A. Whitworth, a retired Navy radioman.

Arthur Walker is charged with seven criminal counts that could bring three life terms in prison plus 40 years. Prosecutors say he passed two documents -- classified "Confidential," the least sensitive category -- from his job as an engineer at VSE Corp., a Chesapeake, Va., defense contractor. The documents deal with the repair schedules for various ships. The prosecution has described them as "critical" secrets, but defense lawyers dispute that.

The prosecution describes Arthur Walker as a plotter and subversive. But even if that is the case, court documents suggest that the Little League coach and father of three is easily led to do things that put him in a bad way.

Walker relatives describe John Walker as being as unpredictable and wild as his older brother Arthur was quiet. But they say that Arthur admired John's boldness, and that Arthur sometimes followed his lead.

Arthur told the FBI that he knew back in the 1970s that John was involved in "something illegal," partly because he had so much spending money. Another tipoff was when Arthur's wife, Rita, repeatedly told of oblique conversations with John's wife, Barbara, that "something is fishy with John." But Arthur said he didn't know what John was doing.

For Arthur, the trouble started in January 1980, one month after the failure of a radio repair business owned by John and himself. Arthur's debts were huge, and John Walker was bailing him out financially.

Arthur Walker later told the FBI he was "truly down in the dumps," and on the verge of tears when John drove him to a Virginia Beach waffle shop and they took a walk. John Walker told his brother about his lengthy dealings with the Soviets, Arthur told the FBI. Arthur said he told his brother, "Now I know where you get your money."

Explaining later to a grand jury why he joined John's scheme, Arthur seemed to babble. He said it was partly his need for money, loyalty to John and confusion. Meekins said John "worked him over" with guilt about his debts to John.

To get hold of classified information, Arthur took a job at VSE, and months later started showing documents to John. But the documents were not good enough.

"Don't waste your time or take the risk or make me take the risk of carrying the documents around," Arthur recalled John saying. "If documents concern old equipment, it's not worth the effort" for the Soviets.

John Walker continually pestered Arthur to get more sensitive material, even after they spent hours photographing documents in the back of John's van, the FBI said. John told his brother to change jobs to get more high level information.

Arthur told the FBI he felt uncomfortable about John's demands, and sometimes showed him documents to "prove to him that I didn't have anything worthwhile."

Arthur has admitted taking $12,000 from John for the documents, and told the FBI the money "hooked" him. But he also said that he gave John back almost half that amount because he felt he was continually in debt to him.

At one point, Arthur Walker said, his brother showed him a half-inch thick envelope filled with money. "If you could get your hands on good information, this is the kind of money you could have," Arthur quoted John as saying.

At one point, he said he told his brother that he got so nervous photographing documents that he was afraid more camera work would bring a heart attack. "No more photographing for me," Arthur said he told John, "I can't do."

John Walker was arrested on espionage charges in the early morning of May 20 in Rockville after allegedly leaving a bundle of documents for a Soviet agent at a spot in rural Montgomery County.

At 6:30 that morning, FBI agents woke up Arthur and started asking him about his brother. At first he lied and said he didn't know anything about spying, but after failing two polygraph tests over the next few days, he began talking. A government source called him "a fountain of information."

Arthur was so talkative in his visits to the Norfolk FBI office that agents told him "it would be in his best interest to contact an attorney as his involvement in espionage activities with his brother was becoming more and more apparent," an FBI memo said.

But Arthur responded that he had "no concern" about a lawyer, the FBI said. He repeatedly signed forms waiving right to counsel.

He kept showing up early for FBI appointments, bringing notes he had jotted down. He showed up at a grand jury hearing in Baltimore without even being subpoenaed, and confessed to espionage.

Arthur later said he misunderstood, and thought the FBI agents and prosecutors had told him he would stand a good chance of facing no charges if he cooperated. Federal Judge J. Calvitt Clarke Jr. ruled last month that FBI agents did not mislead Arthur Walker.

Meekins said Arthur talked so freely with the agents in part to help the government with an assessment of the damage caused by the compromised information.

Even after the FBI arrested him on May 29, he gave the agents an unopened letter he had received from John since John's earlier arrest. Arthur didn't get a lawyer until he needed one for his first hearing.

"A street kid who's been arrested a lot by the time he's 16 knows, 'I don't tell nothin,' " Meekins said. "But a good citizen, an average Joe like Arthur, he spills his guts. He trusts the system and now he's on the short end of the stick."

Meekins said that whatever the outcome of the trial he thinks Arthur will "do what is right," and testify against his brother. Arthur Walker is "mad" at John, but means him no harm, Meekins said. Plea bargain sessions have failed so far, Meekins said.

Yesterday's testimony did not produce startling news, but again dealt with Arthur Walker's extended conversations with the FBI about his activities.

In addition to the two confidential documents Arthur Walker is charged with giving to John, it emerged yesterday that he also may have passed on a more damaging "secret" document that dealt with machinery aboard ships that warns of incoming radar-guided missiles. An FBI agent testified that Arthur Walker "could not recall" whether he had given his brother this document from among VSE Corp. material.