The Supreme Court ruled unanimously yesterday that federal courts may not referee child custody fights between parents living in different states -- a ruling some state officials say will encourage parental child-snatching.

The court said that Congress, in passing the 1980 Parental Kidnaping Prevention Act, did not intend to allow custody battles, traditionally resolved in state courts, to be moved into federal courts -- even when two state custody decrees conflict.

Between 25,000 and 100,000 children were being snatched annually by noncustodial parents, many of whom hoped to relitigate custody in another state.

The 1980 law eliminated that incentive by requiring states to enforce the custody decisions of other states.

"Instructing the federal courts to play Solomon . . . would entangle them in traditional state-law questions that they have little expertise to resolve," Justice Thurgood Marshall wrote for the court. "This is a cost that Congress made clear it did not want the {1980 law} to carry."

The case, Thompson v. Thompson, began in 1978 when Susan A. Clay, a neurologist, asked for a divorce from neurologist David A. Thompson and custody of their year-old son.

After a trial on the issue of visitation rights, Clay was given custody, and Thompson was awarded liberal visitation rights.

Clay then received permission to move to Louisiana in 1980 with sole custody pending a court study. Three months later, Clay filed a petition in that state to enforce the custody decree, and the Louisiana court awarded sole custody to Clay.

But the California court, after completing its review, gave sole custody to Thompson, who then sued in federal court to enforce that order.

The federal court dismissed the complaint, and the 9th U.S. Circuit Court of Appeals affirmed. Other appeals courts, however, have ruled that federal courts can step in to resolve such impasses.

Marshall, upholding the 9th Circuit's ruling, noted that Thompson "did not attempt to enforce the California decree in a Louisiana state court before he filed suit in federal court."

Marshall rejected Thompson's "unspoken presumption" that the states are "unable or unwilling to enforce the provisions of the act.

"This is a presumption we are not prepared, and more importantly, Congress was not prepared, to indulge," Marshall said. "Should state courts prove as obstinate as {Thompson} predicts, Congress may choose to revisit the issue."

Briefs by four states and several child advocacy groups urged that federal courts be allowed to intervene when state courts conflict.

The court also ruled yesterday that judges may be sued by court employees for alleged civil rights violations, rejecting arguments that allowing such lawsuits would undermine judicial independence.

Justice Sandra Day O'Connor, writing for a unanimous court, said judges enjoy absolute immunity from lawsuits for judicial actions but not administrative actions.

The decision involved Illinois Judge Howard Lee White, who demoted and then fired probation officer Cynthia A. Forrester.

Under Illinois law, White had authority to hire and fire probation officers.

Forrester sued White for sex discrimination, and a jury awarded her $82,000. But the trial judge ruled that White was entitled to judicial immunity from a civil damages suit, and a divided panel of the 7th U.S. Circuit Court of Appeals agreed.

O'Connor, reversing that ruling in Forrester v. White, said judges' personnel decisions are the same as employment decisions made by executive branch officials. Absolute immunity, O'Connor said, is justified only when there is a clear danger of interference with the judicial function. The case was sent back to the lower courts.

Lawyers familiar with immunity questions said the ruling would apply to federal judges in most instances.