Chart: Apple’s U.S. smartphone share doubled in September by Philip Elmer-DeWitt @FortuneMagazine October 21, 2013, 11:11 AM EDT E-mail Tweet Facebook Google Plus Linkedin Share icons Source: Monthly Market Pulse. Click to enlarge. FORTUNE — Driven by the iPhone 5S and 5C, Apple’s AAPL share of U.S. sales surged in September to 39% of the smartphone market and 34% of the broader mobile phone market, according to a report issued Monday by Seoul-based Counterpoint Research. “In spite of supply constraints and just three weeks of sales, iPhone 5s raced to become the bestselling smartphone model in the US in September,” said Counterpoint research director Tom Kang. “The key reason for this sudden jump in sales can be attributed to the big wave of upgrades due in this launch month from Apple’s huge installed base of iPhone users in US. This depicts a healthy replacement cycle and user base growth trends for Apple despite the array of Android devices launching from tens of OEMs every month.” Counterpoint’s press release did not describe the survey’s methodology.