Evan Williams, the co-founder and chief executive of Twitter, is stepping down to lead product strategy at the company, Twitter announced on Monday. Dick Costolo, the chief operating officer, will succeed Mr. Williams.

The company, which announced the change in a blog post, is navigating a period of rapid growth and beginning to build an advertising business.

In the two years that Mr. Williams has been chief executive, Twitter has grown to 300 employees from 20; the service’s registered users have ballooned to 160 million from three million; and the company has started to generate revenue.

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The news was unexpected. Though technology blogs had raised the question of whether Mr. Costolo would eventually become chief executive since he was hired a year ago, Mr. Williams had always insisted that he wanted the job. But on Monday, he and Mr. Costolo both said the decision was Mr. Williams’s choice and that he wanted to get back to his roots, working on the product. The moves were effective immediately.

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Mr. Williams said in an interview that he realized he wanted to focus on product strategy while he was working on Twitter’s new Web site, introduced during the last month. “I was reminded that maybe that’s where my passion and skills lie,” he said, “and I couldn’t focus on that as much as I’d like.”