It was 12 episodes into the first season of "Family Matters" before he made his entrance, in oversized glasses, saddle shoes, and pants hiked up to his pectorals.

And in a matter of seconds -- about the time it might take to say "Did I do that?" in his trademark high-pitched whine -- Steve Urkel was a great, big hit. Everyone's favorite nerd.

"The first time America laid their eyes on this young man when he came onto the show, it was love," noted Reginald VelJohnson, who plays Carl Winslow, patriarch of the clan that the show was supposed to revolve around.

But, on that fateful evening in late 1989, the studio audience went so wild over the Winslows' geeky next-door neighbor, played by then 12-year-old Jaleel White, that "Family Matters" producers immediately made him a regular -- and increasingly, the show's main focus. Soon, there were even Urkel dolls for sale.

Not since ultra-cool Arthur "Fonzie" Fonzarelli's "Happy Days" takeover had a breakout character scored such a coup.

Although White, now 21, does not believe this warning will apply to him, making an indelible impression as either a series star or a breakout backgrounder can also have a down side. As veteran producer Danny Jacobson said recently, "As far as I'm concerned, Ted Danson will always be Sam Malone, and Henry Winkler will always be Fonzie."

If TV history repeats itself, after "Family Matters" signs off tonight -- with the conclusion of a two-part finale that was made, before CBS canceled the series, as the season finale -- White may have a tough time getting viewers to see him as anyone but Steve Urkel.

A number of TV actors have become victims of their own success. While they may continue to land acting jobs, often they cannot duplicate the success of that one signature character.

Winkler, for example, starred in the 1982 movie "Night Shift" (directed by "Happy Days" co-star Ron Howard), and more recently, had a role in "Scream," but his self-produced 1994 Fox series, "Monty" (in which Winkler played a conservative TV host), was short-lived. Mostly, Winkler is a successful television producer (the syndicated series "Sightings" and last year's Showtime series "Dead Man's Gun," in which Winkler acted.)

Many members of the "Happy Days" (1974-84) cast have, in fact, either moved to the other side of the camera (Anson Williams, who played Potsie, now directs) -- or disappeared from Hollywood's radar screen (Donny Most, who played Ralph Malph).

And Howard, who starred as Richie Cunningham (and, as a child, as Opie on "The Andy Griffith Show"), went on to become a successful director ("Cocoon," "Parenthood," "Apollo 13"). So did Rob Reiner ("Stand By Me," "A Few Good Men"), Meathead on "All in the Family," and Penny Marshall ("Big," "A League of Their Own"), Laverne on "Laverne & Shirley." Marshall's co-star, Cindy Williams, hasn't fared as well, and David L. Lander, who played the popular Squiggy on the sitcom, appeared in David Lynch's short-lived 1992 series "On the Air" and little else.

And except for John Travolta, the sweathogs on "Welcome Back, Kotter" -- including Ron Palillo, who played red-hot Arnold Horshack -- went underground.

The long list of actors who seem to have been hampered by typecasting includes: Jim Nabors ("Gomer Pyle"); Bronson Pinchot (Balki on "Perfect Strangers"); Valerie Harper, whose Rhoda Morgenstern became a hit on "The Mary Tyler Moore Show," then moved on to her own "Rhoda" sitcom; Gary Burghoff (Radar O'Reilly), one of many members of "M*A*S*H" whose careers stalled; and Danson, who, after "Cheers," did well in the NBC miniseries "Gulliver's Travels," but flopped big time in his expensive CBS series "Ink." (Shelley Long has also had a rough time.)

White believes he'll escape that list, in part because on "Family Matters," he occasionally played other characters, including Myrtle Urkel, Steve's cousin from Biloxi.

"I'm not going to let Hollywood limit me," White said last summer, as his series was about to make what proved to be a disastrous leap to CBS, after dominating Friday nights for eight seasons on ABC. "I know what I can do. I did those other characters convincingly," said White, who, out of his Urkel get-up, doesn't much resemble his famous character.

Tonight's finale has Urkel cruising through the universe -- and floating weightlessly -- on a space shuttle. He's aboard to test a gravity-manipulating device, which causes a midspace mishap. But naturally, Urkel returns safely and resumes his plans to finally marry his beloved Laura Winslow (Kellie Williams).

During the show's early years, VelJohnson was said to have been bitter about being upstaged by Urkel. But even he appears to have made peace with the interloper -- who, VelJohnson acknowledged last summer, "was one of the direct reasons that the show had such longevity."

To be sure, Urkel had his fans -- including Alvin Poussaint, the professor of psychiatry at Harvard Medical School who served as "The Cosby Show" script consultant and deemed Urkel a groundbreaking African-American character. "They didn't used to portray black kids as nerds, because it implies you have some brains," Poussaint once said.

White -- who grew nearly a foot during the show's run -- is clearly proud of Urkel. But he's also apparently happy to be parting company with the character that brought him fame and fortune, and also dictated that he not grow facial hair, change his hairstyle, or even lift weights.

In two years, White, who has nearly finished his first screenplay, expects to have a degree in film and television from UCLA. He'd like to be involved in all aspects of show business -- writing, producing, and directing, as well as performing. But he says his next acting role won't be soon -- or anyone vaguely resembling Urkel.

"If you ever see me doing that character again, put a bullet in my head and take me out of my misery," White told TV Guide recently. "Call Dr. Kevorkian. Because I'm obviously trying to commit suicide and just don't know how to do it."

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