In an effort aimed at tightening control of the U.S.-Mexican border, the government will soon begin replacing millions of Border Crossing Cards with state-of-the-art documents that use compact-disc technology to store information.

The replacements, called "laser visa" cards, will have security features intended to make them much more difficult to counterfeit than the vulnerable Border Crossing Cards, about 5.5 million of which are estimated to be in circulation, officials say. Like the old cards, the new ones are to be issued only to Mexicans.

But while officials tout the card-replacement program as a valuable tool in the fight against immigration fraud, immigrants' rights groups question the increased cost to applicants and the government's ability to handle the huge caseload. And the very idea of the new cards is sending chills up the spines of some libertarians, who fear that it means "Big Brother" is making inroads on the southwestern border.

For 40 years, Border Crossing Cards have been issued to Mexicans who live along the 2,000-mile border and need to cross frequently for business or family reasons. The cards allow travel up to 25 miles inside the United States and stay for up to 72 hours. They do not permit employment on the U.S. side.

At least five versions of the card have been issued, all but the latest with no expiration date. Many have decades-old photographs and are easily used by impostors.

Over the years, the Border Crossing Card also has become one of the most counterfeited U.S. documents, allowing untold thousands of illegal immigrants to enter the United States from Mexico, officials say.

The problem prompted Congress to write into the 1996 immigration legislation a mandate for a secure new machine-readable card with "biometric identifiers," such as fingerprints.

Under the rules implementing that law, Friday was the last day that Mexicans could apply for the old border card issued by the Immigration and Naturalization Service at ports of entry. Starting April 1, the State Department will take over the program and begin issuing the new laser visa.

In the interim, the INS will issue temporary cards to qualified Mexicans who need to travel to the United States but do not already have a Border Crossing Card, the agency said Friday.

When presented along with the I94 form that foreign visitors are required to fill out, the new laser visa will allow Mexicans to travel inside the United States and stay for the same periods permitted by the B1 and B2 tourist and business visas, State Department officials said.

All Border Crossing Cards will expire on Sept. 30, 1999, the officials said.

The State Department plans to issue the first laser visas at the U.S. consulate in Ciudad Juarez, across the Rio Grande from El Paso. Within the next several months, they are to become available at the U.S. Embassy in Mexico City and all U.S. consulates in Mexico.

The credit-card sized laser visa will show the holder's name and photograph on the front, and the back will have a mirror-like "optical stripe," which can hold a lot of information and be read by a laser device, much like a compact disc player. Embedded in the optical stripe will be the person's biographical data, digital photograph and fingerprints and a State Department "control number," officials said.

The information will show up on a computer screen when an inspector at the port of entry swipes the card through a machine as if it were a credit card. If the digital photo does not match the one on the front of the card and the face of the person presenting it, the inspector will know it is fake, INS spokesman Donald Mueller said.

The laser visa will cost $45, compared with $26 for the Border Crossing Card, and will be valid for 10 years, he said.

Joel Najar, an analyst for the National Council of La Raza, an advocacy group for Americans of Hispanic descent, said immigrants' rights advocates worry that many poorer Mexicans may not be able to afford the new card. He said there are also concerns that the laser visa may be paving the way for a concept dreaded by libertarians: a "national ID card" for U.S. citizens. INS officials dismissed such fears. Linking a travel document used only by Mexicans to a national ID card for Americans is "kind of a leap," Mueller said. "It's technology that's available to the credit card companies and anybody who wants to pay for it."