The New York Stock Exchange received a dual blow as seat prices dropped below $1 million and American International Group, one of its biggest companies, said it will also list its shares on an all-electronic market.

The $975,000 sale of a seat, or membership, was the first below $1 million since 1995. It came as the Securities and Exchange Commission rewrites rules governing trading and the NYSE’s chief executive officer, John Thain, develops a hybrid market to mesh electronic and manual trading – changes that may undermine the NYSE’s 80% market share in its listed stocks.

“To the extent that markets dislike uncertainty, that may be a factor,” said a spokesman for the exchange, Ray Pellecchia.

The sale was announced after Archipelago Holdings Incorporated, an electronic market based in Chicago, said American International, the world’s largest insurance company, will “dual list” on the Archipelago Exchange. The chief executive of American International, Maurice “Hank” Greenberg, has criticized the auction market on the NYSE, where his company has its primary listing.

“AIG firmly believes in the benefits of electronic trading,” Mr. Greenberg said in a statement. American International is the ninth-largest American company by market capitalization.

The dual listing is largely symbolic, because a company typically doesn’t need to be listed on a particular market in order for its shares to trade there. Yesterday, 89,600 shares of American International traded on Archipelago. Almost 4.4 million shares of the company changed hands on the NYSE.

“I don’t comment on the marketing stunts of other markets,” said Mr. Pellecchia.

Archipelago said American International is paying $10,000 to become the first company to dual list on the four-year-old market.

“We’re very thrilled to have someone like that show confidence in our new program,” said the 56-year-old head of Archipelago’s corporate client group, Sam Long. “It validates what we’re doing.”

There are currently 1,366 memberships at the NYSE, which is a not-for-profit organization. The NYSE is looking at reducing the number of seats outstanding and changing its legal structure, Mr. Pellecchia said. He declined to elaborate. Mr. Thain has discussed the possibility of trading derivatives, such as options, to boost the exchange’s revenue.

NYSE seat prices have plunged 35% in a year, and by more than half since their August 1999 peak of $2.65 million. The bid for a seat is $950,000, and the offer is $1.15 million, the exchange said.