NYSE Group shares fell below $64.25 for the first time, erasing the New York Stock Exchange’s gains as a publicly traded company in the eight weeks since it bought Archipelago Holdings.

The stock fell $2.65, or 4%, to $63.75 in New York Stock Exchange composite trading, its eighth straight daily drop.

NYSE Group shares debuted March 7 at $67, on a first trade by Chief Executive Officer John Thain, and closed at $87.53 a week later.

Investors lost enthusiasm for the 213-year-old NYSE after rival Nasdaq Stock Market took a 15% stake in London Stock Exchange Plc last month, making the first move to expand overseas. The head trader at Philadelphia based Greentree Brokerage Services, Warren West, said some shareholders began to question the NYSE’s prospects after the Chicago Mercantile Exchange reported lower-than-expected first quarter earnings.

“There was a lot of excitement for the deal and a number of people believed the NYSE, with the combination of Archipelago, might not have that much competition,” Mr. West, a former trader at the NYSE, the New York Mercantile Exchange, and the Chicago Board of Trade, said. “But the rest of the world didn’t sit still.”

The stock also came under pressure after NYSE Group said shareholders including UBS AG and Goldman Sachs Group planned to sell at least $1.6 billion in stock this week.

NYSE Group went public by swapping some of its shares for Archipelago stock on a one-for-one basis. Archipelago closed at $64.25 on March 7, its last day as an independent company.

Shares of NYSE Group soared to close at $80 in their first day as investors bet the combined company would dominate stock trading and profit would quadruple by 2007.

Even with the decline in NYSE Group shares, the Archipelago transaction has more than tripled the value of memberships that used to represent seats on the exchange. A membership that sold for $1.62 million before the acquisition was announced April 20 is now worth about $5.48 million based on yesterday’s closing share price. Each membership was converted into about 80,177 NYSE Group shares and $370,000 in cash.

Last month, NYSE Group Chief Financial Officer Nelson Chai said the company will save $200 million by cutting expenses over the next two years.

At their peak, shares of NYSE Group fetched 73 times the company’s 2006 projected earnings of $192.4 million. That made it the most expensive securities exchange among the 16 that are publicly traded around the world.

Shares of other American exchanges also have fallen over the past two months. The decline may signal that a rally in the stocks since 2004 has gotten ahead of the companies’ growth prospects.

“There are certainly opportunities for exchanges going forward, but I don’t think those justify the high price level that we are seeing for exchanges today,” said Steve Swanson, president and chief executive officer of Automated Trading Desk, a Mount Pleasant, S.C.-based brokerage.

Shares of the Chicago Mercantile Exchange have dropped 8.1% since April 24, the day before the world’s biggest futures market reported first-quarter earnings that fell short of analysts’ estimates because of lower trading fees. The stock has risen 12-fold since its initial public offering in 2002.

Shares in the Chicago Board of Trade fell 19% since March 31, reducing the company’s gain following its initial public offering in October to 79%.