WASHINGTON — Government employees who are singled out for arbitrary, irrational or even vindictive treatment by their supervisors will find no relief in the Constitution's guarantee of equal protection, the Supreme Court ruled on Monday, unless the mistreatment is due to discrimination on the basis of race, sex or another protected category.

The 6-3 decision, with an opinion by Chief Justice John G. Roberts Jr., rejected the argument that an individual employee who is not the victim of group-based discrimination can nonetheless suffer a denial of equal protection within the meaning of the 14th Amendment.

A Supreme Court decision eight years ago had accepted the theory that an individual can comprise a "class of one" for equal protection purposes.

That decision, Village of Willowbrook v. Olech, about a property zoning dispute, appeared to mark a new departure in equal protection doctrine. But the question soon arose whether the principle it announced applied to government employment as well as to government regulation. On Monday the Supreme Court said no.

The "class-of-one theory of equal protection" was "simply a poor fit in the public employment context," Roberts said.

"To treat employees differently is not to classify them in a way that raises equal protection concerns," the chief justice said, adding: "A challenge that one has been treated individually in this context, instead of like everyone else, is a challenge to the underlying nature of the government action."

In a dissenting opinion, Justice John Paul Stevens objected that the majority "carves a novel exception out of state employees' constitutional rights." There is a "clear distinction between an exercise of discretion and an arbitrary decision," he said. Justices David H. Souter and Ruth Bader Ginsburg joined the dissent.

The case, Engquist v. Oregon Department of Agriculture, No. 07-474, was brought by an Oregon state employee, Anup Engquist. Engquist, a native of India, was dismissed from the laboratory job she had held for 10 years. She filed a federal lawsuit claiming, in addition to discrimination on the basis of race, sex and national origin, that she had been dismissed simply for "arbitrary, vindictive, and malicious reasons" by a supervisor who disliked her.

The jury rejected her claims of group-based discrimination, but accepted her "class of one" theory and awarded $425,000 in damages. The 9th U.S. Circuit Court of Appeals, in San Francisco, overturned the verdict on the ground that the class-of-one theory did not extend to government employment. The Supreme Court affirmed that ruling.