Twitter, which has flourished thanks to tools built by outside developers, is taking more of those tools under its own wing. In a move that is sure to rattle its developers, Twitter has agreed to acquire Atebits, the start-up that makes the Tweetie apps for using Twitter on Mac computers and iPhones. The acquisition price was not disclosed.

This signals a new strategy, as Twitter makes its first foray into providing a mobile and desktop client itself. On Friday, Twitter also announced that it helped Research In Motion build an official Twitter app for BlackBerrys.

In the past, Twitter has focused on its Web site, twitter.com, and left the development of Twitter mobile clients up to start-ups like Tweetie, Twitterrific and UberTwitter.

Ahead of Chirp, Twitter’s first developer conference next week, developers have been getting nervous that Twitter plans to build or buy more apps for itself, which could put them out of business.

Fred Wilson, an investor at Union Square Ventures and a longtime Twitter board member, stoked those fears in a blog post in which he wrote that many third-party Twitter services, including mobile clients like Tweetie’s, are features that Twitter should offer itself. Instead, he recommended that developers focus on other kinds of services, like business tools, analytics or gaming.

In a recent interview, prior to the acquisition announcement, Twitter’s chief executive, Evan Williams, had a similar message.

“There’s some misunderstanding around platforms,” Mr. Williams said. “There’s both a natural win-win relationship between a platform provider and third-party developers, and there’s a natural tension.”

When Twitter realized it needed its own search engine, it bought Summize, which made one. “Since we’re still evolving, that may happen more,” Mr. Williams said. The Atebits acquisition is a prime example.

Twitter, which raised $100 million in September, has the cash to go on a shopping spree. It cannot hire engineers quickly enough to support the explosive growth of the company, so buying other features is a way to grow quickly.

The acquisition is Twitter’s third. It previously acquired Summize and Mixer Labs, which helps developers build location-based services.

Atebits was founded by Loren Brichter, a software developer who used to work for Apple on the iPhone. It sells for $2.99, is the most popular mobile Twitter client and won an Apple Design Award last year. Twitter will offer it for free and rename it Twitter for iPhone.

On the Mac computer version, Tweetie runs ads from Fusion Ads, an ad network. Twitter has said that it will consider some sort of advertising in the near future.

“I think it hit that perfect combination of power and simplicity,” Mr. Brichter said last year in a talk at Stanford University.

“I really stumbled into a big steaming pile of gold when I wrote Tweetie,” he said. Thanks to Twitter, that pile just got a lot bigger.