Mark Lennihan/Associated Press

Last year, Lane Becker and his wife, Courtney Skott, plotted out the costs of owning a car versus renting one through Zipcar, the popular car-sharing service.

They ultimately decided to give away their car. Mr. Becker, who is 39 and lives in San Francisco, said he had few regrets, despite some difficulty securing a Zipcar on weekends, when he competes with other customers.

“It’s a lot easier to rent than to own in a city these days, and Zipcar is an easy way to do it,” he said.

But on Wednesday, Avis Budget Group, the car rental conglomerate, announced that it was buying Zipcar for about $500 million. And that has Mr. Becker and some other “Zipsters,” as Zipcar customers are known, worried that the company’s communal cachet could be tarnished by a corporate behemoth.

“Please don’t let them screw it up,” Mr. Becker said.

For Zipcar, based in Cambridge, Mass., the deal represents perhaps an inevitable evolution for a company that has been more successful as a collectivist concept than as a profit-making venture (though it announced in November that it would post an annual profit for the first time).

Zipcar, which will operate as a subsidiary of Avis, should realize significant savings on things like vehicle purchases and insurance, while being able to tap Avis’s fleet to meet demand on weekends, when it is often short of cars.

For Avis, the purchase represents a new direction in a fiercely competitive car rental market, and an about-face for Ronald L. Nelson, the company’s chairman and chief executive, who had resisted entering the car-sharing segment.

“I’ve been somewhat dismissive of car sharing in the past,” Mr. Nelson said Wednesday morning in a phone call with analysts. He said he had come to the realization that car sharing could complement Avis’s more traditional car rental business and help it unlock new business opportunities abroad and with younger consumers. Avis’s rivals, Hertz Global and Enterprise Rent-A-Car, already offer hourly rental services that compete with Zipcar.

Shares of Avis Budget Group closed up about 5 percent on Wednesday, at $20.77. Shares of Zipcar closed up nearly 48 percent, at $12.18.

Avis said it expected savings of $50 million to $70 million a year from combining the two companies.

“Avis Budget’s existing infrastructure, scale and experience with managing multiple brands make it uniquely positioned to accelerate the growth and profitability of Zipcar,” Mr. Nelson said in prepared remarks. “At the same time, we are committed to retaining the elements of the Zipcar brand and culture that have allowed Zipcar to achieve such rapid growth and success.”

Avis paid $12.25 a share in cash, a 49 percent premium over the closing price of Zipcar on Monday. (The price, however, is well below Zipcar’s value in April 2011, when it went public at $18 a share.)

Among the beneficiaries will be Zipcar’s early investors, including the tech titan Steve Case. He and his investment fund own about 19 percent of Zipcar’s outstanding shares.

The idea for Zipcar dates to 1999, when a 42-year-old woman named Robin Chase learned about car sharing from a friend who had just returned from Berlin. A mother of three with an M.B.A. from the Massachusetts Institute of Technology, Ms. Chase wrote up a business plan and secured financing. Zipcar was started in 2000.

Environmentalists have embraced the idea of car sharing from the start, but Ms. Chase marketed Zipcar more on the idea of convenience, with the slogan “Wheels When You Want Them.” The company grew quickly but fitfully, and Ms. Chase was forced out in 2003.

Her successor as chairman and chief executive, Scott Griffith, expanded Zipcar across the country and into Europe. A flat, hourly rate that covers gas and insurance for customers proved to be alluring: the service now has more than 760,000 members, with locations in 20 metropolitan areas in the United States, Canada and Europe, as well as on many college campuses.

“Since the founding, I knew that Zipcar would end up being the winning model for drivers, and that car rental companies would eventually have to come around and adopt our technology and approach,” Ms. Chase said in an e-mail on Wednesday.

She said she expected the deal to improve Zipcar’s profitability, but added, “Like all big company acquisitions of small companies, there is worry that they won’t be able to do what it takes to succeed in the new economy: more innovation, more customer participation, more thinking outside of the box.”

Zipcar fans praise its convenience, its plentiful urban locations and its friendly customer service. Victor Neufeld, a media executive who lives in Manhattan, said renting a car in New York City was always an annoyance before Zipcar came along.

“Every time I need a car for two hours, I go online and get a car within two blocks of my home,” said Mr. Neufeld, who declined to give his age. “I also like it because originally I heard about it through word of mouth. I did not learn about it through advertising or some other marketing. It was very organic.”

Maggie Marquis, 31, who lives with her husband in Chicago, said she started using Zipcar about six months ago and was generally pleased. “Just today, I decided that I wanted to run out and do some errands, and decided it would be easiest and cheapest to get a Zipcar,” she said.

But Ms. Marquis said she was surprised by “small hints of disorganization.“ The company sent her multiple membership cards and would not allow her to sign her husband up for the same account, and she said she often received text messages about her rentals even after she had returned a car.

She said she hoped the influence of a larger corporation would “take their service to a new level of sophistication.”

Several other Zipcar fans said they, too, hoped there would be benefits to the Avis deal, particularly more cars available on weekends.

Claire Frisbie, 31, who lives in Brooklyn, said she liked Zipcar for many reasons, but that one of the biggest was the ability to avoid the bureaucracy of traditional car rental companies: the waiting, the insurance, the added fees.

“I wouldn’t want it to make it more difficult,” she said of the deal. “I would be worried that the easy breeziness of it would be gone.”