At the end of the first quarter, there were 136 million active iPhones in the U.S., new data released on Friday by Consumer Intelligence Research Partners (CIRP) shows (PDF). The user base is up from 132 million at the end of 2016 and a little over 80 million two years ago.

CIRP tracks Apple’s (AAPL) total iPhone user base each quarter. The company’s data is extrapolated across the U.S. based on a survey of American Apple customers it conducts each quarter. The latest survey was conducted over three weeks in April.

While Apple’s iPhone growth is good news for the tech giant, CIRP noted that the company’s growth rate has slowed. In 2015, for instance, Apple routinely added about 10 million new U.S. users. Now, it’s slowed to a few million each quarter.

In a statement, CIRP partner and co-founder Mike Levin said that Apple’s slower growth is likely due to many factors, including “the lengthening ownership cycle, a diminishing number of first-time buyers, and increased Android loyalty.” He added that Apple’s apparent interest in its Services business, where it’s investing heavily in music, video, and original content, could be due in part to its own analysis that it has a finite amount of iPhone user growth in critical markets like the United States.

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Still, Apple has done a fine job of getting customers running on its latest hardware and upgrading to new units every few years. In fact, the entire American iPhone user base consists of iPhones that were released within the last 30 months, according to CIRP. The two-year-old iPhone 6 and iPhone 6 Plus account for a third of the U.S. user base. The most recent Apple handsets, the iPhone 7 and iPhone 7 Plus, account for 26% of the current American iPhone user base.

Looking ahead, CIRP didn’t say whether Apple’s user base would plateau. However, in its fiscal second quarter earnings report this week, Apple revealed that iPhone unit sales slipped during the period. The tech giant said the 1% year-over-year decline was due to customers hearing about upcoming iPhones and deciding to wait to buy one of those handsets rather than a currently available device.

Apple is rumored to be planning three new handsets this year, and they’ll likely be released in the fall.