Mexican truckers, however, indicated they didn't need any defense. They praised the House vote on Tuesday that was meant to close the border to them: It also means U.S. truckers would be frozen out of Mexico.

The House voted 285-143 Tuesday to bar the U.S. Transportation Department from issuing safety permits that would let Mexican trucks operate throughout the United States.

The Bush administration said Wednesday it would fight to block the measure in the Senate.

Mexican Economy Secretary Luis Ernesto Derbez told reporters Mexico ``would not be left any choice'' but to retaliate if the measure takes effect.

``These measures would affect a number of imports that we are receiving from the United States in quantities considered equivalent to the losses for Mexico in not permitting the trucks to pass,'' Derbez said.

``It could be, say, to halt imports of fructose (corn sweetener),'' he added. Growing imports of the sweetener have devastated the Mexican sugar industry, so an import ban would please many Mexicans.

The threat of restriction also might pit the politically powerful U.S. corn industry against U.S and Mexican truckers who are fighting a more-open border.

``The majority of people in the United States don't want Mexican trucks to go there, and we told our president that we don't want to go, either,'' said Manuel Gomez, president of Mexico's main trucker's association.

``Neither are we interested in having U.S. trucks come to Mexico,'' he added in an interview on Wednesday.

Unrestricted truck traffic through both countries was supposed to start last year under the North American Free Trade Agreement, which took effect in 1994.

An international mediation panel ruled that the Clinton administration, under pressure from U.S. trucking unions, had violated the treaty by refusing to enact the agreement. That created friction with the United States' No. 2 trading partner.

In a widely publicized gesture of goodwill, Bush had said he would let Mexican trucks deliver throughout the United States as of next January. They currently are restricted to a 50-mile zone north of the Mexican border where U.S. and Mexican trucks swap trailers or unload cargo.

Powerful forces on both sides of the border are joined in alliance of fear of cross-border competition in trucking.

``We feel our sector is too weak to be up against the American trucking sector,'' said Sotelo, owner of Fletes Sotelo, which has 200 trucks that cross the border.

While U.S. truckers fear competition from low-wage Mexican drivers, Gomez, in Mexico City, said low wages ``are the only advantage we have against the United States now.''

``From that point on, everything is more expensive for us: diesel, highways, the price of the trucks, the tires, the parts,'' he said.

Gomez's National Chamber of Cargo Transportation is the legally recognized representative of Mexico's 8,000 trucking companies and 154,000 owner-operators. Gomez said about half are formal members of the chamber.

The chamber has urged Mexican President Vicente Fox to seek suspension of the NAFTA clause on trucking, saying the current system works well.

The House action is meant to legally accept the treaty while effectively blocking it by refusing safety certificates even to perfectly safe trucks.

U.S. legislators indicated the by the time the Senate considers the measure, it might be altered to simply require tougher safety inspections.

Many of Mexico's 375,000 freight trucks have been in service for 15 to 20 years, compared with an average of five years in the United States.

Industry leaders say it would cost billions of dollars over the next decade to bring the Mexican fleet to U.S. standards, though many defend the safety of their vehicles.

Truck insurance agent Fortino Garcia, 37, said there was no use fighting the U.S. decisions.

``It's their country and we can't do anything about it,'' said Garcia, pecking out insurance forms on a manual typewriter. ``They own the world.''