More than 3 million copies of OS X Mountain Lion have been downloaded since its debut last week, Apple said today.
"Downloads of Mountain Lion have exceeded three million in four days, making it the most successful OS X release in Apple's history," the company announced in a statement Monday.
OS X Mountain Lion went on sale early Wednesday. Mountain Lion is available only from the Mac App Store; unlike last year, Apple is not offering the upgrade on a USB flash drive.
Apple waited longer than last year to boast of its latest upgrade's download success. A day after OS X Lion's launch in July 2011, Apple issued a press release claiming it had distributed more than 1 million copies of OS X 10.7 from the Mac App Store in the first 24 hours.
The company's numbers mean an average of 750,000 copies were downloaded daily, less than Lion's first-day tally.
There is only scant evidence to corroborate Apple's assertion.
Last week, advertising network Chitika said its data showed Mountain Lion powered 3.2% of all active Macs 48 hours after the operating system's availability. Chitika's percentage translated into approximately 2.1 million of the 66 million Macs in use that Apple claimed during a presentation at its annual Worldwide Developers Conference seven weeks ago.
By comparison, Windows 7 accounted for about 2.3% of all PCs running Windows 10 days after its Oct. 22, 2009, launch, according to Web analytics company Net Applications.
Most of the Mountain Lion copies downloaded thus far would have gone to paying customers, since owners of new machines have reported they've been told by Apple that the redemption codes for the free upgrades due them under the "Up-To-Date Program" would not be delivered until 72 hours after they submitted their requests.
Up-To-Date provides a free copy of Mountain Lion to people who purchased a new or Apple-refurbished Mac equipped with Lion starting June 11. Some customers complained -- many on Twitter -- of delays in receiving their redemption codes.
Mountain Lion can be downloaded from the Mac App Store, and can be installed on some, though not all, Intel-based Macs currently running its two immediate predecessors, Lion and Snow Leopard.
The $19.99 price is a third less than last year's Lion, and half what Microsoft will sell its Windows 8 Pro upgrade for starting Oct. 26.
OS X 10.8, the numerical designation for Mountain Lion, has not relinquished the No. 1 spot on the Mac App Store's bestselling paid app list since it popped to the top last Wednesday.
Gregg Keizer covers Microsoft, security issues, Apple, Web browsers and general technology breaking news for Computerworld. Follow Gregg on Twitter at @gkeizer, or subscribe to Gregg's RSS feed . His e-mail address is firstname.lastname@example.org.