He enters, almost always, just before the dead of night gives way to dawn. He enters, almost always, a yellow or beige house, through an unlocked window or door. And then, almost always, he walks to the bed where his victims sleep and shoots them in the head.

He is the Night Stalker.

The Night Stalker is the latest serial killer to terrorize Los Angeles, setting a pattern only to break it.

For a time, his habit of entering through unlocked, sometimes open, windows and doors inspired the name the Walk-In Killer. But the pattern did not hold: In one of the killings attributed to him, the victim, 30-year-old student Tsai-Lian Yu, was dragged from her car and fatally shot.

The Los Angeles Herald-Examiner coined the name "Night Stalker" in a banner headline on its front page. The Los Angeles Times called him the Valley Intruder. Some police officers called him the 818 Killer because he had chosen all his victims within the 818 area code of the San Gabriel and San Fernando valleys.

But last week, police say, he went to San Francisco and fired bullets into the heads of a sleeping couple who would have been entitled to think that the Valley Intruder was someone for southern Californians to worry about. Southern Californians breathed more easily: The killer had migrated north.

Then, last Sunday, someone entered a house in Orange County just before 3 a.m. Bill Carns, the 29-year-old owner, was shot in the head and his fiancee was raped. Police investigators, who are witholding information about a signature piece of evidence the killer leaves behind, had no doubt: the Night Stalker had returned.

His victims are young and old, men and women and children, Caucasian and Oriental. He does not always shoot them. Some have been beaten to death. Some have been stabbed. Two have had their throats slashed. Not all of his victims have died; even some who were shot in the head -- including Carns, listed in critical condition today at Mission Community Hospital -- have survived.

Some of the survivors have described their attacker to police. He is, said Commander William Booth of the Los Angeles Police Department, a Caucasian with a tan complexion, brown curly hair and brown eyes. He stands 6 feet to 6 feet, 2 inches tall and weighs about 150 pounds.

His teeth are widely spaced and badly stained. A dentist who told law enforcement agencies that he had treated a man closely resembling a composite drawing of the Night Stalker has turned over the patient's records to authorities.

Last week, Los Angeles County Sheriff Sherman Block told a news conference that the Night Stalker has been linked firmly to 14 murders, the first on March 17, and 19 other assaults. Authorities now say they are investigating the possibility that he also was responsible for a series of child molestations in the San Gabriel Valley.

Today Los Angeles police impounded an orange Toyota station wagon linked to the attack on the Orange County couple. Booth said an anonymous caller tipped police about the location of the car, which had been stolen last week in Chinatown. Its owner said the car had contained literature from his church, which has since received several crank calls from someone who refuses to speak.

Police have sent copies of the composite drawing to gas stations and convenience stores. Leaflets telling citizens how to protect themselves from the Night Stalker were made available today at local police stations. The LAPD's Booth said, "I don't know that you can see much discernible pattern" that would warn police where the killer will look for his next victim.

But Dr. Alfred Coodley, professor emeritus of clinical psychology at the University of Southern California's medical school, and a consultant to the criminal division of the Los Angeles Superior Court, sees a pattern.

"It's true that in the past, serial killers have seemed to select a rather definite kind of person," Coodley said. "They looked for an elderly person, or a child, or someone with a particular kind of hairdo. This one, on the surface, does not seem to have done this . . . . He has selected a whole family."

Coodley said he believes that the man now notorious as the Night Stalker was so abused as a child that he developed into "a borderline personality: very impulsive, poorly controlled and unpredictable." Coodley surmises that the killer is gripped in a "sadistic rage" triggered by some recent event in his life and that his travels to San Francisco and Orange County may have repeated some childhood move.

The "compulsive, driven" killings, Coodley said, bring the Night Stalker "a sick gratification. But that rapidly disappears, and it needs to be repeated."

In Los Angeles, people are shutting and locking windows, no matter how hot it gets. Some residents say they are buying guns. And many seem newly conscious of the color of their houses -- wondering whether the pattern, such as it is, will hold.