Apple's growth rate in terms of Mac shipments in the first quarter outpaced that of the other top five U.S. PC makers, a research firm said Wednesday.

Apple shipments soared by 32.5% to 1 million units, compared with the 762,000 Macs shipped in the same quarter a year ago, Gartner said. Apple ended the quarter with a 6.6% market share that placed it No. 4 among the top vendors in the U.S.

Gartner rival IDC also released a PC market report on Wednesday that had Apple in the No. 4 spot with 6% of the U.S. market. IDC pegged Apple's growth rate at 25.1%.

"Apple enjoyed strong retail sales, and there were indications that Apple showed decent growth in the professional market as well," Gartner said in a statement.

Apple has been making solid gains in the U.S. market since switching to Intel's x86 platform in 2006 from IBM's PowerPC. Within the business market for client computers, Apple last year tripled its market share to 4.2%, though its growth was limited mostly to enthusiasts and small work groups, according to Forrester Research. PCs running Microsoft Windows accounted for 95% of the market last year, a drop of nearly 4% from 2006.

Among the companies that have introduced the Mac in the work environment is IBM. About 100 researchers in IBM Labs are testing Macs as their main computers instead of Lenovo ThinkPads.

Within the United States, Dell was the largest computer maker in the first quarter, followed in order by Hewlett-Packard, Acer, Apple, and Toshiba, according to Gartner. PC shipments overall increased by 3% from the same period a year ago to 15.2 million units.