OSHKOSH, WIS., NOV. 5 -- A rapt jury and wall-to-wall spectators listened this afternoon as a prosecutor and a defense attorney traded opening shots in the trial of an Oshkosh man accused of illegally seducing a woman who claims to have at least 18 personalities -- most of whom, the prosecution contends, had no idea the woman was being seduced.

His profile to the television cameras that stood lens-to-lens along the first row, Winnebago County District Attorney Joseph Paulus told the jury the state intended to prove that Mark Peterson, a 29-year-old married grocery store worker, this summer broke a Wisconsin law that forbids having sex with someone too mentally ill to monitor his or her own conduct. The jury would see, Paulus said, that Peterson had been warned about the young woman's condition -- and that despite that warning, he "intentionally manipulated a mentally ill woman into having sex with him in the front seat of his car in a small park here in Oshkosh."

The charge is sexual assault in the second degree; it carries a possible penalty of 10 years imprisonment.

The 27-year-old woman was prepared to testify, Paulus declared, and in her testimony the jurors would listen to the voices of the personalities who knew about the sex and the personalities who did not. "You'll meet 'Franny,' who's about 30 years old, considered to be the mother hen of all the other personalities, and controller of them all," he said. "You'll meet 'Emily,' 6 years old, childlike, as any 6-year-old, a person who likes to eat candy."

Paulus promised they would also met "Sam," who at one time expressed himself solely through the noises and crouching of an animal; and "Jennifer," the 20-year-old personality the prosecutor said was in control of the young woman's body while she and Peterson were having sex in the car. And they would also meet the young woman whose name and date of birth first appeared on the criminal complaint last August -- a young woman who will be referred to here as "S." In a sexual assault case, convention dictates that victims' names go unpublicized.

That has been nearly the only convention to show up since the inception of this apparently unprecedented case, which has already included a preliminary hearing in which three separate personalities were sworn in on the stand. Defense attorney Edward Salzsieder had ventured forth this morning with a motion no criminal defense attorney is normally able to contemplate: If Wisconsin law prevented him from inquiring into the sexual history of the woman claiming rape, he said, then he ought to be allowed to pursue the sexual history of "Franny" and "Ginger" and anybody else who have been in what S. has called "the dark place" -- the place where her personalities go when another has taken charge.

Judge Robert Hawley, a young man with wire glasses who had already alerted the jury that he expected to swear in at least three personalities, looked momentarily taken aback. "We're trying to split some very fine hairs here, Mr. Salzsieder," he said. "I do find that the rape shield law applies to her and all her personalities combined."

So Salzsieder lost his motion, and when he stood for his opening statement he acknowledged immediately that Peterson and S. had in fact had sex one morning last June. He said the jury might think what Peterson had done was foolish, or immoral, but that Peterson had not intentionally broken any law -- that the jury would see that Peterson had a brief affair with a young woman he did not understand to be mentally disabled.

"I guess I call this the case of 'perceived consent,' -- science fiction, fact, or hoax,' " Salzsieder said.

Back at the defense table, his face expressionless, sat Mark Peterson, a weak-chinned man in a cardigan sweater with a lot of complicated ideas banging around him in a crowded court of law.

Psychiatric books were lifted and displayed. Movies that made popular entertainment of split personality were mentioned before the jury: "Sybil," "The Three Faces of Eve." A Wisconsin psychiatrist talked both about multiple personality and about how he believed it was possible that the person called S. did not know until afterward that her body had participated in sex with Mark Peterson.

"Later on, the other personalities did tell her what happened," said the psychiatrist, Pakistani-trained Inam Haque. "This is the mystery of the personality disorder -- that S. herself was not aware of what was going on, but the other personalities would be aware of what was going on."

S. herself appeared to have suffered a blackout during the sex, Haque said.

Then who, Paulus asked, was occupying her body?

"Jennifer" was, Haque said. The more mature "Franny," he said, had been told by Peterson to disappear.

"Was 'Franny' also in the dark place?" Paulus asked.

"Yes," said Haque.

It is this kind of imagery that has pulled a lot of faraway excitement to the mid-Wisconsin town of Oshkosh, where a local reporter was grinning today about being interviewed by the BBC, and a Hollywood production company woman in a bright yellow suit said her people were working on the rights to S.'s story but had not obtained them yet.

The notion of multiple personality has been invoked in criminal trials before -- Kenneth Bianchi, who pleaded guilty 11 years ago to seven murders in the Los Angeles "Hillside Strangler" cases, claimed to be a multiple personality until his testimony was debunked by psychiatrists who said he was faking.

But this appears to be the first prosecution in which the state claims that the victim is a multiple personality, and that others have an obligation to treat that person as mentally ill if they are made aware of the disease.

"I think this would be breaking new ground," Paulus said this afternoon as reporters clustered around him during a break in the proceedings. "Right now there's a common mind-set in the country about what constitutes mental illness. ... We'd be making an extension, saying people afflicted by a disease like this, a dissociative disorder, can be under the protection of mental illness, and therefore their rights can be protected."

The trial is also an obvious showcase for psychiatric theories of multiple personality disorder, a diagnosis that remains controversial despite the growing numbers of mental health experts who have come to accept it. Cases of multiple personality were reported a century ago, but interest in the idea waned in the early 1900s, and it was not until 1980 that the American Psychiatric Association gave formal recognition to what psychiatrists now refer to as MPD.

Although some psychiatrists have heatedly disputed both their theories and their data, the physicians who have written most extensively about MPD now describe it as a kind of coping mechanism that develops in people who are subjected to trauma so terrible that they cannot bear it -- usually children, the psychiatrists say, who are abused and develop separate "selves" to manage the unendurable emotions aroused by the abuse. These selves may be fragments of personality, the MPD advocates say, or they may be full-fledged individuals so different in their expression that they might speak in different accents, react differently to medication, write with different hands.

"We used to think it was very rare," said University of Louisville psychiatry professor Leah Dickstein, one of the psychiatrists who accept the newer theories of multiple personality disorder and treats MPD patients, "and we realized that many people, for example, were misdiagnosed -- women were misdiagnosed schizophrenics, and the men were misdiagnosed sociopathic."

Schizophrenia, Paulus told the jury today, was one of the diagnoses given to S. as she moved from treatment to treatment in search of some relief from the mental difficulties that had plagued her for many years. "She had voices in her head," he said. "She had periods of amnesia where she had no idea where she'd been, or what she'd just done."

It was not until 1990, Paulus said, that S. received a diagnosis of multiple personality disorder from psychotherapists who had been treating her for four years. She had continued her therapy, he said, but still sometimes shifted rapidly from one personality to another; indeed, Paulus said, the day she met Peterson she was fishing and went through three different personalities while catching a single fish.

Peterson struck up a conversation with S. at the park where she was fishing, Paulus said, and later looked up her name in the telephone directory and came to her home. "He was pushing for a date," Paulus said. S.'s neighbors warned Peterson repeatedly that she was a multiple personality and that he must not take advantage of her, and Peterson himself had already watched S. take on the personalities of "Franny" and others, Paulus said.

When Peterson showed up the next morning at S.'s house, Paulus said, he asked for the personality he had met as "Franny." "He suggested they go for breakfast at a local restaurant," Paulus said. He said 'Franny' agreed -- but that the neighbors then reminded her that moments earlier, when she was in the personality of S., she had eaten a bowl of breakfast cereal.

They went out for coffee instead, Paulus said, and there they talked at some length about S. and her multiple personalities. "All he had to do to get to the personality he liked was to call upon that individual," Paulus said. They got in Peterson's car, Paulus said, and as they were driving, Peterson asked Franny to "go away" so that "Jennifer," a younger personality he had met in the restaurant, might emerge to take her place.

"He proposed that they have sex," Paulus said. He said that Peterson and "Jennifer" were not entirely alone during the sex act -- that "Emily," the 6-year-old personality, kept making her presence known through a voice clear enough that Peterson knew what was going on. "He told her to go back where she came from," Paulus said.

When the sex was finished, Paulus said, Peterson asked "Jennifer" to "go back," and "Franny" to take her place. "There was no conversation about what had happened," Paulus said. "No conversation about the sex, no cuddling, no kissing, no holding hands." He said Peterson drove the woman home, and that he insisted "Emily" was to keep the secret to herself.

"But 'Emily' had been told in therapy that when an adult touches you in a place where you wear underwear, you tell an adult," Paulus said. "You don't keep it a secret." "Emily" told the neighbor what she had seen, Paulus said, and it was an irate S. who later brought charges against Mark Peterson for violating laws meant to protect the mentally disabled from sexual assault.

He did not repeat for the jury the remark S. recalled during the preliminary hearing, but the local papers reported it at the time: The day after the sex act, S. testified then, Mark Peterson called and asked to speak to "Franny." "I said, 'How do you think I feel, you son of a bitch, after what you did to 'Jennifer' and 'Emily'?" S. testified. "And I hung up."