The latest figures from comScore show that Apple gained 0.7 points in the three-month span concluding in August. Apple's iOS and Google Android were the only two platforms in the top five that saw their share increase.
Google continues to extend its lead over rivals, bolstered by Android's availability on a number of major manufacturers with a variety of form factors. Android saw a 5.6 point increase from the May period, bringing its total domestic share up to 43.7 percent in August.
The gains of Google and Apple came mostly at the expense of Research in Motion, which saw its share slide by 5 points from May to August. RIM's BlackBerry line now represents 19.7 percent of smartphones in the U.S., good for third place.
Microsoft continues to struggle to gain traction with its Windows Phone platform, as its share dropped by 0.1 percent in the latest survey . Its share of 5.7 percent was good for fourth place, ahead of Nokia's Symbian, which slipped 0.3 points to 1.8 percent total.
ComScore's figures show Apple on the verge of cracking 10 percent of all mobile subscribers in the U.S., when traditional cell phones are included. Apple's 1.1 point gain in August brought it to 9.8 percent of all mobile subscribers in America.
Apple's total share of mobile subscribers is behind Motorola (14 percent), LG (21 percent) and Samsung (25.3 percent). But among those, only Samsung saw growth from the May period to August, rising by 0.5 points.
comScore's numbers are based on a survey of more than 30,000 U.S. mobile subscribers. The study found that 234 million Americans age 13 and older use mobile devices, and 84.5 million people in the U.S. owned smartphones during the three months ending in August, up 10 percent from the May survey.