More than 60 percent of tipped workers are employed in food service, but there are other categories of subminimum-wage workers as well, including some that cannot count on any tip income.

Au pairs are young people from other countries who come to the United States on cultural exchange visas to live with families and care for their children. They do not have a typical employee-employer relationship. Under a federal program, au pairs are paid a stipend of about $195 a week, as well as receiving room and board from the host family.

Here are more industries and job titles that the Fair Labor Standards Act exempts from the federal minimum wage.

Students

Full-time students under 20 who are employed by retail or service establishments, in agriculture, or by colleges can be paid a minimum wage of $4.25 an hour during the first 90 consecutive calendar days on the job. Students in vocational education can be paid that wage as well.

High school students who are at least 16 years old and work part-time can be paid between 75 percent and 100 percent of the minimum wage as long as the student is enrolled.

Employees on Small Farms

This category includes agricultural employees who are immediate family members of their employer, or who live locally but not on the farm. And it includes minors who are hand harvesters and are employed on the same farm as a parent.

Seasonal and Recreational Employees

Workers in jobs that last only part of the year — during the holiday season, say, or over the summer — at amusement parks, ice cream shops, summer camps and other attractions make up the bulk of this group, which tends to skew younger. But there’s also a chance that older workers can be placed into this category and be taken advantage of. “It might mean that others who are not teens getting paid less than minimum wage,” Mr. Cooper said.