NUEVO LAREDO, Mexico -- The new $100,000 Volvo tractor-trailer pulled into a spotless terminal here, just shy of the Texas border, and unloaded $250,000 worth of Sony electronics bound for U.S. customers.

The Mexican-owned truck -- world class, with a satellite tracking system and technology to record speed and mileage -- was banned from hauling its cargo all the way into the United States despite the commitment to eliminate trade barriers in the North American Free Trade Agreement (NAFTA), which has regulated U.S.-Mexican commerce since 1994.

The U.S. ban on Mexican trucks is the most hotly contested trade issue between the countries. It is a sore point in a relationship otherwise dramatically improved over the past decade, particularly since President Vicente Fox took over here last December.

The vast majority of the goods made or assembled in Mexico and exported to the United States -- which range from Sony televisions to Gap jeans and are worth $140 billion a year -- are delivered by Mexican trucks to the border area, but no farther. American truckers haul them the rest of the way, and on to stores and warehouses around the United States.

Underscoring the importance of the issue in both countries, Fox will travel to Detroit on Monday to meet with James P. Hoffa, head of the International Brotherhood of Teamsters, which opposes allowing Mexican trucks and drivers to travel U.S. highways.

The Teamsters, with 1.4 million members in the United States and Canada, is barred from organizing workers in Mexico and would like to change that. But the union's public stand on Mexican trucks revolves around safety. The Teamsters warn that untrained Mexican teenagers will be driving along U.S. highways in 80,000-pound rigs -- many of which are unsafe, the union says -- if the ban is lifted.

"NAFTA has cost us American jobs," Hoffa has said. "It must not cost us American lives."

President Bush, who has struck up a friendly relationship with Fox, said that opening the border to Mexican trucks and complying fully with NAFTA would be the legal and right thing to do. But the House of Representatives recently voted to retain the ban on Mexican trucks, raising the specter of a trade war. Bush called this vote wrong and warned the Senate on Wednesday that he would veto such a measure if it reached his desk.

Highway safety has been a powerful argument against lifting the truck ban. About 5,000 people die each year on American roads in accidents involving trucks. More trucks on the roads could mean more accidents, and the idea of drivers who speak little English and are unfamiliar with U.S. roads adds a new layer of concern.

Others insist there are ways to minimize the risk, mainly by admitting only Mexican drivers and trucks with proper operating permits and by increasing border inspections. Many Mexicans contend that beneath the expressions of safety concerns lies old-fashioned protectionism, aimed at preserving the jobs of Teamsters.

"This fight is about Mexican drivers' salaries being lower than Americans'," said Miguel Quintanilla Rebollar, a trucking company owner and past president of the main Mexican trucking association, the National Chamber of Cargo Transporters. "They are worried about us taking their jobs."

Noting that American companies are already recruiting drivers in Mexico, he said that worry may be justified.

"It's not fair," added Quintanilla, surrounded by his fleet of brand-new trucks. "Americans are only looking at the old trucks, and we have trucks that are as good as any in America."

Mexican trade official Luis de la Calle said 1 million Mexican cars are on U.S. roads and Mexican pilots fly into U.S. airports. Why would trucks be any different, he asked.

"In life, in all things, there are risks," he said. "All we are asking is that our companies, our trucks and our drivers are given the chance to comply with all U.S. standards, that we are not discriminated against because we are Mexican."

Mexican officials said they hope the issue can be resolved amicably. If the ban continues, they said they would consider retaliating with new tariffs on U.S. imports, such as the 500,000 tons of imported American fructose sweetener bought in Mexico every year.

Brian McLaughlin, a U.S. Transportation Department official involved with the issue, said that Mexican trucking has made "huge improvements" in the past five years and that there are "very reasoned arguments about responsibly opening the border." But it has been difficult to focus members of Congress and others on "very technical, data-driven proposals" when "folks are nervous about big trucks," he said.

Clouding the issue further is that opposing Mexican trucks is a way to vent anger over the relocation of American plants and jobs south of the border and actually represents a backlash against free trade.

Bret Caldwell, a Teamsters spokesman, agreed that Americans may be tired of losing jobs to Mexico, such as the 400 that shifted recently when a Mr. Coffee plant in Cleveland was closed. But he said that proposals to modify the trucking ban rely heavily on the word of Mexican companies about their trucks' conditions and their drivers' records.

Mexico has no national data bank of driving violations comparable to the United States', so a truck driver could be repeatedly stopped in different parts of the country without any of the police officers knowing of the other stops. If that trucker then drove to Texas and was caught speeding, there is no reliable system in place to check his record in Mexico, Caldwell said.

"They are calling us xenophobes and protectionists," he added. "The fact is, Mexico hasn't met its end of the bargain."

Mexican and U.S. government officials said they are working on coordinating a system to better cross-check driver safety information. The plan calls for hiring more than 100 new Transportation Department employees to audit safety reports from Mexican trucking companies and for more than doubling the number of federal and state inspectors near the border -- for a total of 490.

Here at ground zero of free trade, the busiest border truck crossing in North America, there is a different view of Mexican trucks.

"They are great for Laredo," said Tony Leja, a lumberyard worker, just over the Rio Grande on the U.S. side. "Ten years ago, this was a ghost town. There was nothing to do but watch TV. Then NAFTA happened, the trucks started coming, and look around."

He pointed to a skyline of cranes building warehouses, hotels, hospitals and housing developments.

Laredo, Tex., across the border from Nuevo Laredo, has become one of the fastest-growing cities in the United States, thanks in large part to NAFTA. Nearly 10,000 truck crossings are registered every day over the bridges linking these two cities, many of them the same trucks going back and forth.

"Those bridges are the jewels of South Texas," said Rafael Garcia, director of the World Trade Bridge inaugurated last year by Bush, then governor of Texas, and Ernesto Zedillo, then president of Mexico.

As Garcia stood in his second-story office on the bridge, overlooking nearly nonstop truck traffic, he said every five-axle truck pays a $13.75 bridge toll. That means $32 million a year for Laredo. Five years ago, he said, Laredo could not afford to pave its streets.

But the trucks Garcia sees outside his window are not long-haul rigs going to Detroit or St. Louis. Because of the ban, they are old workhorses hauling cargo from one side of the border to the other -- from arriving Mexican trucks on one side to departing U.S. trucks on the other. They stay in the 20-mile commercial border zone, spending most of their time in traffic at the border booths.

Critics say that even these old trucks, which frequently fail inspection, will slip into the United States if the ban is lifted, because no number of inspectors can cope with the huge flow of border traffic.