hone and iPad, Forstall had been considered a favorite to replace Jobs as CEO after Jobs' death last October. More recently, Apple observers pegged Forstall, 43, as the next in line behind Cook.
Along with marketing chief Phil Schiller and design guru Jonathan Ive, Forstall anchored the company's top tier of executives, frequently speaking at Apple's heralded product unveilings to showcase the ins and outs of operating-system upgrades.
Many felt he was the most logical choice to continue Jobs' legacy of innovation at Apple, where 70% of revenue now comes from the mobile devices that iOS powers.
"He was as close to Steve as anybody at the company," Andy Miller, former head of Apple's iAd group, told Businessweek last year. "When he says stuff, people listen."
The Apple press release did not say what Forstall plans to do when he leaves ...view middle of the document...
Ive, Bob Mansfield, Eddy Cue and Craig Federighi "will add more responsibilities to their roles," according to the announcement.
Cue, the senior vice president for Internet software and services, will take over voice-controlled assistant Siri and Maps, two of iOS's most prominent features. Federighi, the vice president of Mac software, will now oversee both Mac and iOS.
Almost universally lauded for its smooth, easily navigated design, iOS was dinged by many last month for a wonky Maps app and other problems. Replacing Google Maps for the first time as the system's default tool, users found the map product incomplete and sometimes comically wrong in many locations throughout the world.
Some critics said iOS 6 didn't offer many significant advancements over its predecessors, while Google's Android system continues to evolve more rapidly.
Monday's changes, particularly Forstall's departure, came as a major surprise. The only comment from Cook in the announcement didn't shed much light on them, either.
"We are in one of the most prolific periods of innovation and new products in Apple's history," Cook said in a written statement. "The amazing products that we've introduced in September and October, iPhone 5, iOS 6, iPad mini, iPad, iMac, MacBook Pro, iPod touch, iPod nano and many of our applications, could only have been created at Apple and are the direct result of our relentless focus on tightly integrating world-class hardware, software and services."
Forstall did not speak at last week's unveiling of the iPad Mini. But he took the stage at September's iPhone 5 event, detailing the aforementioned operating-system updates.
Forstall has been with Apple since 1997 and, in addition to guiding iOS, is one of the original architects of the company's desktop and laptop software, Mac OS X. He was responsible for several versions of the software, most notably Mac OS X Leopard in 2007.
Before Apple, he worked with Jobs at his short-lived NeXT, the computer company he ran between stints at Apple.