Net Applications, which also tracks Web users' operating systems, said that the increase in Safari's share has been matched move for move by a climb in Mac OS X use. In January, the combined PowerPC- and Intel-based Mac OS X share was 6.2 percent, up from December's 5.7 percent. "Both Safari and Mac OS X are heading in the same direction -- up," Vizzaccaro said.

Windows XP still has an overwhelming lead in operating systems, however, with 85 percent. Microsoft's Internet Explorer accounted for 79.8 percent of the browser market in January.

Windows Vista, which debuted to businesses in November but only hit retail Jan. 30, had just 0.2 percent share by then, according to Net Applications.