Mike Segar/Reuters A Zipcar outside the NASDAQ on the car-sharing service’s stock debut.

In an encouraging sign for the larger I.P.O. market, shares of the car-sharing service Zipcar soared nearly 67 percent on their first day of trading.

The company, best known for its nationwide fleet of affordable rental cars, priced its initial public offering on Wednesday at $18 a share, according to a regulatory filing — well above the previously expected a range of $14 to $16 a share. Zipcar, which raised $174.3 million in its offering, also sold more shares than expected, offloading 9.7 million shares, according to the company.

The stock opened at $30 on the Nasdaq market, trading under the ticker “ZIP.”

Zipcar’s warm reception is another sign that the I.P.O. market, which was moribund during the financial crisis, has begun to thaw.

So far this year, the United States market has welcomed 40 I.P.O.’s, with proceeds exceeding $16 billion — more than double the corresponding period for 2010, according to Renaissance Capital.

Still, analysts caution, investors are not sinking money into I.P.O.’s indiscriminately. Of the three other companies that priced on Wednesday night, two priced below their expected range, while the other, Arcos Dorados, the world’s largest McDonald’s franchise chain, priced above its range and raised $1.25 billion.

“Investors are not looking at these I.P.O.’s with starry eyes,” said Kathleen Smith, a principal at Renaissance. “Investors are being very selective; Zipcar is trading well because it is establishing a new market and they are profitable in a number of their early markets.”

Zipcar is a capital intensive business, with more than 8,000 vehicles across the United States. The service, which is most active in top metropolitan areas like Manhattan, Boston and Chicago, requires users to sign-up for an annual membership. Once enrolled, drivers can reserve cars, often located in public parking lots, and pay by the hour, with insurance and gas included.

The company, founded in 2000 and based in Cambridge, Mass., has proved popular, with 560,000 members.

But Zipcar is not yet profitable. According to a recent filing, Zipcar posted a net loss of $14.7 million in 2010 and a loss of $4.7 million for 2009. The company, which has accumulated losses of $65.4 million, said it did not expect to be profitable this year.

Still, revenue is growing. Sales rose 41.9 percent to $186.1 million last year. In its filing, the company said it planned to use the proceeds from the I.P.O. to pay down debt and for general business expenses.

Part of the reason Zipcar is saddled with losses is its aggressive campaign to build its network and thwart rivals, like Hertz, which started its own car-sharing program, Connect, in 2008. Zipcar is now in Canada and Britain, where it recently purchased Streetcar, a London-based competitor.

“Competition is the biggest worry, but if they can stay ahead of the game, this could stock could do very well,” Ms. Smith of Renaissance said. Hertz’s Connect is available in eight cities, while Zipcar is in more than 200 markets.

Venture capital investors in Zipcar include Revolution Living, led by the founder of AOL, Stephen M. Case; Benchmark Capital Partners; Greylock Partners; and Smedvig Capital. Revolution, which did not sell shares in the offering, owns a 21.5 percent stake.

This post has been revised to reflect the following correction:

Correction: April 14, 2011

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