Apple Inc. (NASDAQ:AAPL) continues to dominate U.S. smartphone market share, ending 2013 with a 45% unit share, according to the latest NPD Group report. The company benefited from the 21% growth in the smartphone market in the U.S., as consumers bought over 121 million smartphones during the year.

Apple AAPL

U.S. smartphone industry is stable

At the end of 2012, Apple Inc. (NASDAQ:AAPL) had a 44% unit share of the U.S. smartphone market, so it has increased its share by one percentage point. Of course Samsung Electronics Co., Ltd. (LON:BC94) (KRX:005930) is in second place, but it holds just 26% of U.S. smartphone units. Nonetheless, the company also increased its share of the U.S. market, from 24% at the end of 2012.

NPD Group reported that smartphone sales to consumers who earn less than $30,000 a year increased by double the industry rate in 2013, making up nearly a third of all smartphone sales. That ended up being the biggest share of any income bracket. On the other hand, those who earn over $100,000 a year dropped off significantly to only 4%.

Apple’s customers are rich… still

Of course Apple Inc. (NASDAQ:AAPL)’s customers still tend to be the wealthiest, as a third of those who bought an iPhone were considered to be more wealthy than the average customer. In fact, Apple’s iPhone made up 65% of the sales among wealthier shoppers. The company’s handset also did remarkably well among those earning less than $30,000 a year, growing 64% year over year among that income demographic group.

Samsung Electronics Co., Ltd. (LON:BC94) (KRX:005930)’s customers were mostly in that sub-$30,000 income group, which made up 35% of all of its U.S. sales. That’s double the percentage of wealthier consumers who bought the company’s smartphones.

Apple Inc. (NASDAQ:AAPL) will likely continue to face challenges as sales to lower income customers and those in emerging markets continue to be the main sources of growth. NPD group also noted that most of the growth during 2013 in the U.S. came from prepaid devices, which grew by 68% during the year, making up 29% of the U.S. smartphone market. Currently Apple doesn’t participate in the prepaid market.