The survey conducted in August found that 43 percent of all smartphone owners said they have an Android device, compared to 28 percent who own an iPhone. When looking at users who bought a smartphone in the last three months, Android's share grew to more than half, at 56 percent, while Apple stayed once again at 28 percent.
In fact, Apple's 28 percent share in the U.S. is flat with what Nielsen found in a similar survey in June. In that poll, Android represented 39 percent of smartphone users in America.
Nielsen noted that Apple's market share could "change quickly" in the coming months, as Apple is expected to introduce a new fifth-generation iPhone on Oct. 4. The research firm noted that every new iPhone launch from Apple results in an increase in sales.
With iOS holding steady and Android growing, Research in Motion's BlackBerry lineup fell from a 20 percent share in June to 18 percent in August. When looking at users who bought a smartphone in the last three months, only 9 percent chose a BlackBerry.
Polls have shown for some time now that Android, which is available on handsets from multiple hardware manufacturers on all four major carriers in the U.S., is the largest smartphone platform. But numerous analyses have shown that Apple rakes in more than half of the global mobile phone market's profit.
Nielsen's latest poll numbers show that the domestic smartphone market continues to grow. Though 43 percent of all mobile subscribers in the U.S. had a smartphone as of August, 58 percent of respondents who bought a new device in the last three months chose a smartphone.
"The holiday season and the launch of new devices like the next iPhone could further accelerate smartphone adoption, though this is always tempered by the fact that many consumers are unwilling or unable to break their service contracts before they expire," the firm said.
"In any event, the growing popularity of app-and-media friendly smartphones spells tremendous opportunity for those advertisers, publishers and developers eager to leverage mobile media."