THIEVES who steal a new kind of credit card for an online shopping spree are likely to be disappointed. That’s because a California company has designed a card with an unusual security feature: it works only when it recognizes the voice of its rightful owner.

Enclosed in the card is a tiny microphone, a loudspeaker and a chip with voice recognition chip. To use the card, its owner must speak a password, which the chip compares with a sample recorded on the card. If the voices match, the card emits a set of beeps that authorize the transaction over the telephone or through a microphone on the shopper’s computer. If the voiceprints don’t agree, the card will not beep.

Designed by Beepcard, a company in Santa Monica, Calif., the device is still in prototype form. But one day it may be used to verify ownership over the telephone or the Internet, reducing the cost of fraud for consumers and merchants.

“This card makes it possible to turn a high-risk transaction over the phone or computer into a low-risk one,” said Jon Callas, chief technical officer at the PGP Corporation in Palo Alto, Calif., which develops encryption software used mainly for e-mail.

Credit card fraud is a costly problem for consumers and merchants alike, Mr. Callas said. Merchants pay $2 to $5 on each $100 charged to credit companies, depending on their contracts. “The riskier the credit companies consider the transaction, the more they charge,” he said. Transactions made without the physical presence of a card are considered highly risky. “This system turns a card-not-present transaction into a card-present transaction,” Mr. Callas said.

Beepcard already makes a security card, said Alan Sege, chief executive of the company. This card is used, for instance, by students to gain access to college services online. A student holds the card up to a PC microphone and presses a button on the card, and a series of coded beeps is emitted by a small speaker. “The card calculates a one-time cryptographic signal” that identifies the user to the server and allows access, Mr. Sege said.

The new version of the card uses the same system. “Now it won’t beep unless you authenticate it with your voice,” Mr. Sege said. It could be used with a computer microphone for an online transaction, or over the telephone.

A number of companies are considering using the new card, he said, including Providian Bank.

Mr. Sege hopes that the card will have uses besides authentication. “It’s convenient to have a sound recorder in your credit card,” he said. Such a device might be used as a memory prompt, for instance, to read back a shopping list.

The width and length of the prototype card have been pared to match a credit card. “But it’s still about the thickness of a printed circuit board,” said Nir Dvash, an engineer for the company.

To use the card, a person selects and says a password that is stored in the card’s memory. The system takes into account some variations in a voice to accommodate a cold or background noise. “It has tolerances that can be changed so the user can choose a security level,” Dr. Dvash said. For online shopping at home, for instance, where the risk is low, the user could set the device to accept a wide variation in voice. “But if you go out onto the street you should set it high,” he said, because of the greater risk of being overheard or of theft.

Patrick McDaniel, a senior researcher at AT&T Labs in Florham Park, N.J., who specializes in security, said the new card was a good idea. “The use of encryption for one-time passwords sent in beeps has been around for a decade,” he said, and voice recognition, even longer. “But the combination of the two is novel.”

Yet he pointed to difficulties that must still be overcome. “It will have to be very robust,” he said, to withstand heat, cold and the bumps of back-pocket storage.

Problems could also surface with the voice recognition program. “Accurately identifying the speaker is a difficult task at best,” he said. “Throw in blaring music and you may have a problem.”

David Nahamoo of I.B.M.’s Thomas J. Watson Research Center in Yorktown Heights, N.Y., who has worked in speech technology for 20 years and heads conversational biometrics research at the lab, agreed that voiceprints by themselves were not perfectly reliable. “With voiceprints today, there is a 2 percent error rate of false acceptance and false rejection,” he said.

To get around this problem, the I.B.M. voice authentication system is based not on the limited processing power of a chip on a card, but on the might of a server. It combines voiceprint matching with a rotating set of questions that the user must answer before gaining access to an account.

“A set of random questions is more secure,” Dr. Nahamoo said, to back up biometric identification. “Even if someone eavesdropped and magically got access to your voiceprint, the knowledge won’t be there to answer the questions,” he said.

Even in the imperfect world of voiceprints, Mr. Callas said, the Beepcard may have a chance at success, because of its combination of encryption backed up by voice biometrics. “It becomes convenient and good enough,” he said. “And ‘good enough’ security is extraordinarily important. Most of the world works on ‘good enough’ security.”