Smartphones running Google's Android are the most popular among those buying new handsets, suggest figures.
The survey by market research firm Nielsen shows that in the three months to the end of August, 56% of Americans buying a phone chose Android.
Apple iPhones running its iOS software were bought by 28% of new smartphone buyers.
Nielsen said the figures could change quickly if Apple launches the iPhone 5 next month as expected.
Writing on the Nielsen blog, analyst Don Kellogg said the smartphone market as a whole was growing as more and more people plump for the sophisticated devices over feature phones.
Among those who bought a phone over the last few months, 58% picked up a smartphone. Higher, noted Mr Kellogg, than the 43% seen among all phone buyers Nielsen has tracked since August 2010.
Among all smartphone subscribers, rather than those new phone owners, Google's Android is the most popular platform with 43% owning one. Apple is second with 28% and Blackberry third at 18%. Other platforms, including Nokia, Microsoft, and Samsung are lumped into the remaining 11% share.
Mr Kellogg speculated that the market share mix could see some volatility over the next few months in the run up to Christmas - typically a time when a lot of people splash out on new handsets.
Apple's expected launch of the iPhone 5 could also bump up spending, though he added any expectations about growth should be "tempered by the fact that many consumers are unwilling or unable to break their service contracts before they expire".