WASHINGTON — Most U.S. consumers don't realize Internet merchants and even traditional retailers sometimes charge different prices to different customers for the same products, according to a new survey.

The study, "Open to Exploitation," found that nearly two-thirds of adult Internet users believed incorrectly that it was illegal to charge different people different prices, a practice retailers call "price customization."

More than two-thirds of people surveyed also said they believed online travel sites are required by law to offer the lowest airline prices possible.

The study, expected to be released today by the Annenberg Public Policy Center of the University of Pennsylvania, is the latest to cast doubt on the notion of sophisticated consumers in the digital age.

It said 87 percent of people strongly objected to online stores charging people different prices for the same products based on information collected about their shopping habits.

The Internet empowers careful shoppers to compare prices and features across thousands of stores. But it also enables businesses to quietly collect detailed records about a customer's behavior and preferences and set prices accordingly.

Changing prices is generally lawful unless doing so discriminates against a consumer's race or gender or violates antitrust or price-fixing laws.

Stores aggressively try to retain loyal customers who generate the highest sales while discouraging bargain-hunters, who are less profitable.

First-time buyers at a retailer could see higher prices than a firm's repeat customers, and retailers may not offer discounts to consumers who buy the same brands regularly without even looking at alternative products on the same site.

"It's really murky because companies are so loath to discuss this," said researcher Joseph Turow. "This is a new model of shopping reality. The question becomes, what do people feel is right? Can't more openness be the order of the day?"

The study urged government to require retailers to disclose exactly what information about customers is collected and how the data is used.

Turow found a retail photography Web site charging different prices for the same digital cameras and related equipment depending on whether shoppers had previously visited popular price-comparison sites. He said groceries increasingly offer personalized discounts and coupons based on a person's shopping behavior.

Amazon.com outraged some customers in September 2000 after one buyer deleted the electronic tags on his computer that identified him as a regular customer and noticed the price of a DVD changed from $26.24 to $22.74. The company said it was the result of a random price test and offered to refund buyers who paid the higher prices.

The Annenberg study was based on results from a telephone survey from Feb. 8 to March 14 of 1,500 adults who said they had used the Internet within the past 30 days. The margin of sampling error was plus or minus 2.51 percentage points.