By JOAN E. SOLSMAN

Amazon.com Inc.'s fourth-quarter earnings fell 57%, and the Internet retailer again warned it could post an operating loss in the current quarter.

Shares were down 8.5% at $178 in after-hours trading Tuesday. The stock had hit its highest level ever in October and had since fallen 21% through the close Tuesday.

The company expects $12 billion to $13.4 billion in revenue in the current quarter, while analysts surveyed by Thomson Reuters on average had expected $13.41 billion. It said the operating bottom line could range from a $200 million loss to a $100 million profit.

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In the latest period, operating expenses rose 38%, just outstripping the pace of revenue growth. But that's the lowest percentage increase in the costs in a year.

The company has been on a long, expensive investment campaign to improve its distribution network and digital offerings, but investors have grown increasingly exasperated by how long Amazon opts to sacrifice near-term profit for its goals. In October, the online retailing leader unnerved investors when it reported a 73% plunge in profit for the third quarter and issued conservative bottom-line targets for the latest period.

The company's popular Kindle Fire was a suspected margin pressure point as well. Lately, Amazon has been trumpeting the recent runaway demand for the tablet, its latest model in the popular Kindle family of devices, during the quarter's key holiday shopping season. However, the Fire's profit potential is uncertain, with market researcher IHS estimates the Kindle Fire costs as much to make as its selling price.

However, in the latest period, gross margin rose to 20.7% from 20.3%.

In the latest period, Amazon posted a profit of $177 million, or 38 cents a share, from $416 million, or 91 cents a share, a year earlier. Analysts predicted 17 cents a share.

Net sales increased 35% to $17.43 billion. Excluding currency effects, the growth would have been 34%. In October, the company predicted $16.45 billion and $18.65 billion.

Operating income fell 45% to $260 million. In October, the company forecast it could report anything from an operating loss of $200 million to an operating profit of $250 million.

Write to Joan E. Solsman at joan.solsman@dowjones.com