Google's Android operating system continues to outpace the competition in the U.S., market researchers Canalys and NPD Group have found.

According to Canalys, Google's Android platform was running on 43.6 percent of all the smartphones purchased in the United States in the third quarter. It was followed by Apple's iOS, which captured 26.2 percent market share, and Research In Motion's OS, which tallied 24.2 percent share. Microsoft's mobile OS held 3 percent market share in the quarter.

NPD reported similar results to Canalys.

NPD found that Android was running on 44 percent of smartphones bought during the third quarter, though it claims Apple's market share was 23 percent. RIM was just behind Apple with 22 percent market share, according to NPD. The market researcher asserts that Android's growth over the second quarter--about 11 percentage points--was stolen from "RIM, rather than Apple."

"The HTC Evo 4G, Motorola Droid X, and other new high-end Android devices have been gaining momentum at carriers that traditionally have been strong RIM distributors," NPD executive director Ross Rubin said today in a statement. "And the recent introduction of the BlackBerry Torch has done little to stem the tide."

As disappointing as it might be for Apple to see Google take the top spot in the mobile OS market, the company can find solace in the fact that the iPhone 4 was the best-selling mobile phone in the quarter, according to NPD. In fact, four of the five top-selling handsets were smartphones, NPD said.

The iPhone 4 was followed by RIM's BlackBerry Curve 8500 series and the LG Cosmos, which is a messaging phone not a smartphone. The Android-based Motorola Droid X and HTC Evo 4G rounded out the top five.

As popular as the iPhone has been, it's hard to see how Apple will be able to keep pace with Google. Canalys found that worldwide, Android was running on "more than 20 million units" sold during the third quarter. That figure represents a whopping 1,309 percent gain year over year, compared with the 1.4 million Android-based devices sold in the third quarter last year.

Android's strong third quarter follows a similarly blistering second quarter. During that period, Android was running on 33 percent of all smartphones sold in the U.S., NPD reported at the time. Android was followed by RIM and Apple with 28 percent and 22 percent of the market, respectively.

Going forward, many researchers believe Google's mobile OS will continue to see similar successes. Gartner said in September that it believes Android will own 29.6 percent of the worldwide mobile market by 2014, trailing only Symbian with 30.2 percent share. In a similar report, IDC said it expects Symbian to have 32.9 percent worldwide market share in 2014, while Android will have 24.6 percent share. RIM and Apple would own 17.3 percent and 10.9 percent of the worldwide market, respectively.