SAN FRANCISCO -- Since the late Steve Jobs unveiled the iPhone in June 2007, the innovative smartphone has been the primary driver of Apple's revenue growth.
Now, the long growth run for the company's marquee product may be nearing an end, raising questions about Apple's ability to increase overall revenue.
Sales of the iPhone, which contributed just over half of the company's revenue for the quarter ended in December, rose less than 7%, to 51 million units.
The 6.7% year-over-year growth rate was sharply lower than for the company's prior two periods, as iPhone unit sales rose 25.6% for the quarter ended in October and 20% in the June quarter.
A customer inspects the new iPhone at the Wangfujing flagship store on September 20, 2013, in Beijing.
iPhone revenue rose just 6% during the most-recent period, to $32.5 billion.
And the slowing growth is set to continue, as the company's revenue forecast for the March quarter was well below what Wall Street was expecting.
That forecast, which helped send Apple shares down as much as 9% in after-hours trading, raised the question of whether the iconic tech giant is still a growth company.
The ebbing iPhone growth is especially alarming to Apple investors because it came during the first full quarter of availability for the company's new flagship smartphone, the iPhone 5s.
It suggests that most consumers in mature smartphone markets, such as the U.S. and Europe, who want to own an Apple device already do so.
The much-slower iPhone sales also come at a time when most market analysts forecast that the overall smartphone market is growing at more than twice that rate.
That means Apple is losing market share to Samsung Electronics and other makers of lower-priced smartphones.
If the trend continues, it will signal that a time long feared by Apple stock bulls has come.
Namely, that future growth in the smartphone market -- as with those for PCs and other consumer tech products that came before -- will be driven less by high-priced innovation and more by buyers who are either replacing existing devices or focused on price, not snazzy new features.
The trend may not be as bad as it looks, however, as Apple executives said on a conference call that the weak revenue forecast for the current quarter was impacted by other factors.
For example, the company built a larger inventory of iPhones and iPads for the 2013 holiday shopping season, after being unable to meet demand the year before.
The forecast was also crimped by a stronger U.S. dollar, Apple CFO Peter Oppenheimer said on the call.
The company's iPhone sales rose 40% in Japan, for example, helped by a new agreement with NTT Docomo, that country's largest mobile operator.
Yet a stronger U.S. currency will blunt the revenue benefit of future NTT sales, which are paid for in yen.
Sales of iPads are helping pick up some of the slack, rising 12% in the December quarter versus a year earlier, after coming in flat during the prior period.
Still, the company's North American iPhone business "contracted year-over-year," CEO Tim Cook said on the call, suggesting that the product's growth days in its home market are over.
Without smartphone sales driving growth, Apple will be hard-pressed to meet Wall Street's expectations for an 8.5% annual rise in revenue for the fiscal year ending in September.