STOCKTON, Calif. - Since illegally crossing the Mexican border into the United States six years ago, Ángel Martínez has done backbreaking work, harvesting asparagus, pruning grapevines and picking the ripe fruit. More recently, he has also washed trucks, often working as much as 70 hours a week, earning $8.50 to $12.75 an hour.

Not surprisingly, Mr. Martínez, 28, has not given much thought to Social Security's long-term financial problems. But Mr. Martínez -- who comes from the state of Oaxaca in southern Mexico and hiked for two days through the desert to enter the United States near Tecate, some 20 miles east of Tijuana -- contributes more than most Americans to the solvency of the nation's public retirement system.

Last year, Mr. Martínez paid about $2,000 toward Social Security and $450 for Medicare through payroll taxes withheld from his wages. Yet unlike most Americans, who will receive some form of a public pension in retirement and will be eligible for Medicare as soon as they turn 65, Mr. Martínez is not entitled to benefits.

He belongs to a big club. As the debate over Social Security heats up, the estimated seven million or so illegal immigrant workers in the United States are now providing the system with a subsidy of as much as $7 billion a year.