SAN FRANCISCO (Reuters) - Amazon.com Inc said on Wednesday quarterly net income doubled on a 41 percent rise in revenue, helped by higher operating profit margins.

The online retailer posted second-quarter net profit of $158 million, or 37 cents per share, compared with $78 million, or 19 cents per share, a year earlier. Revenue in the quarter, which is seasonally the slowest, rose to $4.06 billion.

Analysts, on average, have been expecting earnings per share of 26 cents on revenue of $3.95 billion, according to Reuters Estimates.

Seattle-based Amazon, which has been lowering prices on many goods to spur purchases during the U.S. economic downturn, reported a rise in operating profit margin to 5.3 percent of total sales from 4.0 percent a year ago.

Valued at 45 times projected 2008 earnings, Amazon shares trade well above many Internet stocks, including Google Inc, as well as brick-and-mortar retailers with big online divisions like Wal-Mart Stores Inc, Target Corp and Best Buy Co Inc, at 17, 13 and 12 times projected earnings, respectively.

(Reporting by Alexandria Sage; Editing by Braden Reddall)

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