The U.S. Immigration and Naturalization Service (INS) believes that four to six million aliens are living and working illegally in the United States. Every year, the INS makes almost one million apprehensions of undocumented workers, 90 percent of them Mexicans. Fewer than one million individuals are caught, because some persons are apprehended several times in a year. Even though more illegal aliens appear to be employed outside than inside agriculture (see table), they are often assumed to be only an agricultural problem.