Apple has delivered more than three million copies of OS X Mountain Lion in four days, it has announced.
The figures, totalling number of downloads but not sales, make Mountain lion the most successful operating system release in Apple's history.
The upgrade from Mac OS X Lion adds a notification centre, Airplay streaming and deeper iCloud integration into the venerable OS.
"Just a year after the incredibly successful introduction of Lion, customers have downloaded Mountain Lion over three million times in just four days, making it our most successful release ever," said Philip Schiller, Apple's senior vice president of Worldwide Marketing.
The upgrade is available on the Mac App Store for £13.99 - a low price which has likely helped Apple hit the milestone in such short order.