"The Simpsons" virtually tied "The Cosby Show" in national Nielsen ratings Thursday night, and this is one time when a tie can be considered a victory.

Fox's wacky-tacky cartoon clan did better than expected against NBC's six-year-old blockbuster on a pivotal night -- the first time that both series aired first-run episodes since Fox positioned The Simps opposite The Cos earlier this year.

For most shows, the move would be considered a death knell, but "The Simpsons" scored an 18.4 rating and 29 share -- its best showing ever -- against Cosby's 18.5/29, one of that show's lowest.

ABC, which aired an old "Columbo" rerun, earned a 10.6/17 opposite the two powerhouse hits, and CBS logged a 5.4/9 with "Dark Avenger," a feeble movie rushed in as emergency filler when the American League baseball championships ended after only four games.

A rating is the percentage of total TV households, and a share represents the percentage of sets in use at the time. Since Fox had sold "Simpsons" advertising time based on a projected 26 share, the showing was definitely stronger than anticipated.

The feat is made sweeter for Fox by the fact that NBC has 208 affiliates and Fox only 133. A Fox spokesman says NBC has a 10 percent greater coverage area than its upstart competitor, making it an uphill fight for Homer, Marge, Bart and the other members of the Simpson clan.

It was a big night for television generally, with the four-network share of viewers reaching 84 percent. TV sets in 63.7 percent of the nation's 93.1 million TV homes were in use, as opposed to 59.5 percent the previous Thursday -- an indication of how advance hullabaloo about the Simpson-Cosby showdown lured viewers.

In Washington, where Fox affiliate WTTG (Channel 5) normally does better than Fox stations in other major markets, "The Simpsons" put up less of a fight: a 20.3/31 compared with "The Cosby Show's" 24.4/37. But a WTTG spokesman said that in ratings provided by Arbitron, a competing service, the two shows were virtually tied, 24.0/36 for "Simpsons" and 24.1/36 for "Cosby."

Nationally, the "Cosby" rating fell short of its average for the 1989-90 season, a 23.1/38, despite the fact that "Cosby" had the additional advantage this week of what the network calls "a McMillions reveal." NBC and McDonald's are embarked on a month-long promotional contest in which viewers pick up lottery tickets at McDonald's and watch specified NBC shows that night to see if they hold the winning number.

Originally, Thursday's number was to be announced during the telecast of NBC's low-rated sitcom "Grand." But late Tuesday, NBC decided also to announce the winning number during "Cosby," partly to avert a possible humiliating defeat at the hands of the Simpsons.

The half-a-rating-point advantage "Cosby" had over "Simpsons" cost NBC $10 million. That was the prize won by Thelma Bellows, 49, of Kawkawlin, Mich., who held the winning "McMillions" number and saw it pop up during a commercial break.

An NBC spokesman in Burbank insisted that the network was not having a cow over the surprisingly strong "Simpsons" performance. "On the charts next week, 'Cosby' will be ahead," she said, referring to Nielsen's weekly national ranking of shows.

"It was an exciting night of television. We're very pleased with 'Cosby's' performance," the spokesman said. "'Cosby' held its own against heavily promoted competition."

At Fox, there was controlled elation. "The numbers are a little bit better than we thought," a spokesman said. "We were hoping to be a strong second. Everybody's very, very pleased. It's a good day around here."