The portion of people surfing the Web using a Mac has doubled in the past eight months, an Internet metrics analyst said Tuesday, and represents an audience that can't be ignored by Web application developers.

"The amazing story since last summer has been how well the Mac is doing," said Geoff Johnston, an analyst at WebSideStory in San Diego. "For the longest time, Mac hung around 3 percent of the operating systems using the Web. But it picked up around last summer, and has nearly doubled its market share."

Measurements from WebSideStory and rival Net Applications of Aliso Viejo, Calif., put Apple's Mac OS X at close to or just over 6 percent of all machines in the U.S. that connected to the Web last month.

"For the first time since 1999, when we started tracking, the Mac has really made a major push," said Johnston. Since August, the percentage of online Macs running Apple's operating system has climbed from the long-flat 3 percent to 5.6 percent, he said. Net Applications data, which splits the Mac's share between computers running the PowerPC version of Mac OS and those with an Intel edition of the operating system, pegged the total share at 6.2 percent for April.

"Mac has almost doubled," Johnston said, "so you know they're selling a butt load."

There's a correlation, Johnston believes, between the surge and Apple's transition to Intel starting in January 2006, when CEO Steve Jobs announced the availability of the first Intel-powered Macs. Data from WebSideStory's competitor supports that take. Net Applications' data on PowerPC-equipped Macs' share of systems surfing the Web essentially remained stable over the past 12 months; all of the growth, then, came from Intel-powered Macs.

"When you see Mac, or any browser, like Firefox, moving past 5 percent, you just can't turn them away," said Johnston. Web site designers and Web application developers, he said, had better pay attention to the Mac.

Oddly enough, the rise in Macs is both good and bad news to Microsoft, depending on the development division inside the company. "Microsoft's pretty adamant about wanting us to always report the share of Internet Explorer within Windows only," said Johnston, because Microsoft no longer supports a Mac version of IE. "More and more we're going to have to separate Windows from everything else" to accurately calculate Internet Explorer's share of the browser business.

"With the Mac up, IE's losses aren't as bad within Windows only as they are within all operating systems," he said. On the other hand, Johnston pointed out, losing users to the Mac means losing potential Windows customers.

"Macs are starting to erode Microsoft's market share," Johnston declared.

Net Applications' numbers, however, show Windows holding firm. In the first four months of 2007, Windows' overall share of Web-connected computers has stayed steady at around 90 percent. Only the mix of the various editions of Windows has changed. Windows Vista's part of Microsoft's operating system market share, for example, has climbed from just 0.2 percent in January to 3 percent in April, reported Net Applications. That gain on the part of Vista came at the expense of Windows XP and Windows 2000, both which have lost market share this year.

After losing ground last month -- possibly to Vista -- the Mac share rebounded from March's 6.08 percent to April's 6.21 percent, noted Net Applications.