The company posted strong earnings in a brutal climate that has punished nearly every other retailer, online and offline. Amazon’s net profit rose 9 percent, to $225 million, or 52 cents a share, in the quarter that ended on Dec. 31, up from $207 million, or 48 cents, in the same quarter a year earlier.

“We remain relentlessly focused on serving customers with low prices, great selection and free shipping offers, including Amazon Prime,” Jeff Bezos, founder and chief executive of Amazon, said in a statement.

The company’s revenue climbed 18 percent to $6.70 billion, surpassing Wall Street’s expectations. Analysts polled by Thomson Reuters on average expected $6.44 billion in revenue.

After hours, Amazon shares were trading around $56, an increase of more than 13 percent. In the regular trading session, before Amazon’s announcement, shares closed largely unchanged at $50.

Amazon’s robust growth rate far exceeded the overall growth rate of other online retailers during the holidays, which was slightly down, the Web measurement company comScore said.

“We’re particularly grateful for the unusually strong demand for Kindle in the fourth quarter,” Mr. Bezos said, referring to the company’s electronic book reader. It was out of stock for most of the holiday season, but the company encouraged consumers to order it for later delivery.

Amazon released no new statistics on sales of the Kindle, which has been a hit. The company has scheduled a news conference in New York on Feb. 9 to introduce a new version of the device.

Some analysts say Amazon may have benefited from the economic slowdown as people turned to the e-commerce site to find bargains.

“Amazon may be enjoying a Wal-Mart effect, with people trading down to Amazon to get better prices over the holiday,” said Mark Mahaney, an analyst at Citigroup. “Amazon must have dramatically taken market share” from other retailers during the quarter, he said.

But in a sign that Amazon was not immune to the recession, its operating margins fell to 4.06 percent from 4.78 percent, a result of heavy discounting to persuade reluctant shoppers to buy.

Amazon offered a broad estimate for the current quarter and did not make any estimate for the year, as it normally had. It said it expected operating income of as much as $210 million, a 19 percent increase over the first quarter of 2008. At its most pessimistic, the forecast was for a 9 percent increase.

One immediate challenge for Amazon is the liquidation of Circuit City. The electronics chain is emptying 154 stores, which could drive down the prices of items like flat-screen televisions.

In the long term though, the carnage in the traditional retail sector is “incredibly positive” for Amazon, said Scott W. Devitt, an analyst at Stifel Nicolaus. “Amazon has never been in a stronger structural position, and you have a shrinking industry with a high fixed-cost base that is forcing bankruptcies,” he said. “That business has to go somewhere.”

In another sign of how Amazon.com has diversified its business beyond its original product lines of books, music and movies, the sales of electronics and general merchandise in the fourth quarter grew to $2.89 billion, a 31 percent increase over a year earlier. The sale of media items grew just 9 percent in the same period.