Apple's newest operating system dubbed Leopard, was the company's best-selling upgrade, a research group said on Monday.

Sales from the first month of the Leopard Operating System this November were up 32.8 percent over the previous operating system, Tiger released in May 2005, according to NPD Group. The former system is known as Apple Mac OS 10.5, the latter is Apple Mac OS 10.4.

The number of Leopard copies sold was also 20.5 percent higher than Tiger.

Microsoft has experienced launches like this before. Windows ME is a good example. But Apple has yet to stumble, says NPD directing analyst Chris Swenson in AppleInsider, It's really stunning to see Apple have one blow-out OS launch after another.

It's clear that Apple has hit upon the right strategy for rolling out new versions of its OS, Swenson added.

Apple, the maker of the Macintosh computer and iPod digital music player, released Leopard on October 25. The launch had been delayed by four months as Apple focused its efforts on developing the iPhone.

NPD, compiled its data from retails sales, including Apple's online store and It excluded from its count boxed copies of Leopard that were preinstalled on new Macs, just as it did with Tiger and the edition prior to that, called Panther.

Apple, on the other hand, included every possible copy of Leopard when it boasted sales of 2 million during the operating system's first weekend in October.

The company will not report on the quarter from which NPD drew its numbers until late January after the current quarter closes December 31.