Yahoo on Monday said chief executive Terry Semel has stepped down, and company co-founder Jerry Yang has been chosen to take his place, a move that followed increasing investor complaints over anemic stock prices and leadership lapses that left the company in catch-up mode with rival Google.

In announcing the executive changes, the Yahoo board named as president Susan Decker, executive VP and head of advertising. Semel, who had been chairman and CEO, would become a nonexecutive chairman and serve as an adviser to the company.

Semel said in a company statement that he had long talked to the board about the importance of ensuring a smooth succession to senior leadership committed to carrying out Yahoo's stated multiyear transformation strategy.

"As we discussed my future goals and plans, I was clear in telling the board of my desire to take a step back sooner rather than later," he said.

Semel, a former Hollywood executive who has served as Yahoo CEO since 2001, said that the time for change had come. "This is the time for new executive leadership, with different skills and strengths, to step in and drive the company to realize its full potential -- it is the right thing to do, and the right time is now."

Yang, who founded Yahoo 12 years ago with David Filo, said in the same statement as Semel that he was "very excited to be leading our exceptional team of 12,000 Yahoos around the world."

"It's an honor and a great responsibility, and I look forward to the challenge," he said. "My immediate and overarching priorities are to realize Yahoo's strategic vision by accelerating execution, further strengthening our leadership team, and fostering an even stronger culture of winning."

Board director Ed Kozel praised Semel and listed his accomplishments, which included increasing revenue ninefold since 2001 to $6.4 billion in 2006, boosting operating income from a loss to $1 billion last year, and creating more than $30 billion of shareholder value.

Nevertheless, Yahoo has been faced with growing discontent from stockholders who believe Semel had taken the company in a direction that resulted in delays in upgrading its Web search ad system, which was needed to better compete with Google. In addition, Semel was blamed for the company's weak stock price. Yahoo shares fell about 38% last year and have yet to recover.

Yahoo's ability to raise its stock price is tied to the company's showing progress against Google in the online advertising market. A reflection of its need to catch Google was in Yahoo's announcement in April that it would spend $680 million in cash and stock for Internet advertising company Right Media. The purchase was seen as a response to Google's plans to buy DoubleClick.

The announcement of Semel stepping down was made after stock markets closed. In after-hours trading, however, Yahoo shares were up more than 4% to more than $29 a share.