Correction Appended

In a sweeping realignment of the television industry, Fox Inc. announced yesterday that it would form a new alliance with a broadcast station group that would result in the defection of eight important CBS stations to Fox.

CBS, which had already seen Fox steal away the rights to broadcast National Football League games that CBS had held for four decades, now faces the further humbling exercise of having to seek new -- and probably weaker -- affiliates in many of the nation's biggest cities, including Dallas, Detroit, Cleveland and Atlanta.

Three ABC stations and one NBC station will also switch to Fox under the terms of the agreement with New World Communications Group Inc., a company whose controlling shareholder is Ronald O. Perelman, the chairman and chief executive of Revlon Group Inc.

Fox accomplished its move, which clearly took the other networks by surprise, by pledging to invest $500 million in New World, which owns seven television stations and has plans to acquire eight more. Twelve of those 15 would switch their affiliations to Fox. In exchange, Fox would initiate a partnership with New World to produce and syndicate programming for both of the station groups, as well as for other stations.

Fox has affiliates in all 12 cities, but they are less desirable stations with weaker signals. By luring older, stronger stations in those markets, Fox is, in effect, trading up.

The Federal Communications Commission, had no official comment on Fox's move. An agency spokeswoman said the commission was surprised by the announcement.

The Fox network had its beginnings only in 1986; it lacks the distinguished news operations and rich entertainment history of CBS, NBC and ABC. But Fox's owner and chairman, Rupert Murdoch, has been moving the network toward equal status with those competitors, and yesterday's move was his boldest yet. "We are getting closer to parity," Mr. Murdoch said in a telephone interview. "We are about three-quarters of the way there."

Anthony C. Malara, president of CBS's affiliate relations division, said he was stunned when he received the news in a phone call yesterday morning from William C. Bevins, chief executive of New World. Mr. Malara said he asked Mr. Bevins in disbelief, "What are you doing?"

Mr. Malara spent much of yesterday calling stations in the cities affected by the change. "It's a big blow," he acknowledged. "But it's not Armageddon."

He promised that CBS would have new affiliates in each city, though he said that many might have to be weaker UHF -- ultra-high-frequency -- stations, those with channel positions above 13.

The announcement by Fox yesterday set up a scramble for new relationships between networks and affiliates, suddenly making precarious the partnerships that date to the beginning of the television era.

In Detroit, for example, the current Fox affiliate has been owned by Paramount Communications, recently acquired by Viacom Inc., which is in the process of starting its own network. That station had been expected to stay with Fox; now it is likely to align itself with its parent company. If that station is not available, CBS will then have to seek a different affiliate in Detroit -- perhaps the one now linked with NBC. Similar turmoil could arise in many cities affected by Fox's move.

Contracts between a network and its affiliates are for a short period, usually a year or two, and can be broken by either party upon expiration. Because of that, Fox officials said yesterday that they thought they could have all 12 new affiliates in place within 18 months, some of them by fall.

CBS was known to be concerned already about New World's practices, since many of its stations had begun pre-empting CBS programs like its morning news. But the switch to Fox will surely be seen in the industry as another sign of the ascendancy of that network -- which is still not even formally defined as a network by the Federal Communications Commission -- mainly at the expense of CBS. With the New World stations, Fox will improve its total of very-high-frequency stations to about 40 percent of the network, from just 25 percent. These VHF stations, found from channels 2 to 13 -- are most desirable to broadcasters. Higher Ratings Likely

Those totals are likely to translate into improved ratings for Fox, while CBS, depending on which stations it is able to secure in the affected cities, could sustain serious damage to its programs across the board. In a typical season, CBS's overall prime-time rating would be about a 12, compared with about a 7 for Fox. Each rating point represents 942,000 homes.

Among the programs most likely to be affected is the "CBS Evening News," which will lose lead-ins from established local stations in big cities. The Fox stations usually have limited local newscasts and almost none produce an evening local newscast.

CBS could also be hurt in late-night programming. Its "Late Show With David Letterman" will be compelled to follow newscasts or syndicated reruns that are likely to be rated far lower than those on the stations that are abandoning CBS.