SAN FRANCISCO (Bloomberg) - Apple Inc. posted first-quarter profit that disappointed investors and gave a forecast that fell short of analysts' estimates, sending the shares lower and signaling an economic slowdown may curb demand.

Net income rose 57 percent to $1.58 billion, or $1.76 a share, from $1 billion, or $1.14, a year earlier, Apple said yesterday in a statement. Sales gained 35 percent to $9.61 billion.

Earnings failed to top the most optimistic of analysts' estimates. CEO Steve Jobs's forecast also heightened concern that a slowing US economy will cause consumers to cut back on purchases of Macintosh personal computers, iPods and iPhones.

"It's a matter of expectations being out of whack with reality," Daniel Ernst, an analyst with Soleil Securities, said in an interview from New York.

Profit this quarter will be 94 cents a share on sales of $6.8 billion, Cupertino, California-based Apple said. Analysts estimated $1.11 a share in net income, and revenue of $6.98 billion, according to a Bloomberg survey.

Apple fell $18.94, or 12 percent, to $136.70 in extended trading. The stock dropped $5.72, or 3.5 percent, to $155.64 at 4 p.m. New York time in Nasdaq Stock Market trading. The shares have declined 23 percent since reaching a record $202.96 in December.

"We give guidance we believe we have reasonable confidence in achieving," Chief Financial Officer Peter Oppenheimer said in an interview.

Analysts anticipated profit of $1.60 a share on revenue of $9.43 billion in the quarter ended December 29, according to a Bloomberg survey. Profit missed the estimate from Thomas Weisel Partners's Kevin Hunt, whose projection of $1.77 was the highest estimate on Wall Street. Some investors were looking for as much as $2, said Jane Snorek at First American Funds in Minneapolis.

"The whisper numbers were very high," said Ms. Snorek, who helps manage more than $70 billion in assets including Apple shares.

Apple had beaten its forecast in every quarter for three years before yesterday's report, and issued forecasts that missed analysts' estimates in seven of the eight previous periods.

Mr. Jobs, 52, led a revival in Mac sales after incorporating faster chips from Intel Corp. in 2006. Mac shipments topped more than one million in each of the prior 12 quarters, reaching a record 2.16 million in the period ended in September.

Apple sold 2.32 million Macs in the quarter. Apple is working to capitalize on demand for portable machines with the ultra-thin MacBook Air. The $1,799 notebook weighs three pounds and goes on sale next week

Apple posted the biggest gain among US personal-computer makers in the calendar fourth quarter, according to Gartner Inc., a research firm in Stamford, Connecticut. Shipments rose 28 percent, more than double the 9.6 percent increase for market leader Hewlett-Packard Co.

In September, Jobs introduced a new version of the Nano that can play video. He also added the iPod Touch, which uses the same widescreen display built into the iPhone. Apple sold 22.1 million iPods in the quarter, more than the record 21.1 million sold during the 2006 holiday season.

Yesterday's results include iPhone sales in Europe for the first time. Apple said it said sold 2.32 million iPhones in the quarter, adding to the 1.39 million it sold through September.

Apple began selling the handset in the UK, Germany and France in November, after it went on sale in the US in June. The company said the iPhone will go on sale in Asia sometime this year.