WASHINGTON — The U.S. Supreme Court today lifted restrictions on the Navy's use of sonar in training exercises off the California coast, a defeat for environmental groups who say the sonar can harm whales.

The court, in its first decision of the term, voted to allow the Navy to conduct realistic training exercises to respond to potential threats by enemy submarines.

Environmental groups had persuaded lower federal courts in California to impose restrictions on sonar use in submarine-hunting exercises to protect whales and other marine mammals. Environmentalists link sonar to beached whales, internal bleeding around marine mammals' brains and ears, and other damage.

The Bush administration argued that there is little evidence of harm to marine life in more than 40 years of exercises off the California coast.

Chief Justice John Roberts wrote the majority opinion, which was joined by Justices Samuel Alito, Anthony Kennedy, Antonin Scalia and Clarence Thomas.

Six justices agreed with the outcome, although Justice John Paul Stevens did not join the majority opinion. Justice Stephen Breyer would have allowed some restrictions to remain, while Justices Ruth Bader Ginsburg and David Souter said the prospect of harm to the whales was sufficient to justify limits on sonar use.

The court did not deal with the merits of the claims put forward by the environmental groups. It said, rather, that federal courts abused their discretion by ordering the Navy to limit sonar use in some cases and to turn it off altogether in others.

The overall public interest tips "strongly in favor of the Navy," Roberts wrote. He said the most serious possible injury would be harm to an unknown number of the marine mammals.

"In contrast, forcing the Navy to deploy an inadequately trained anti-submarine force jeopardizes the safety of the fleet," the chief justice wrote.

In dissent, Ginsburg said that the Navy's own assessment predicted substantial and irreparable harm to marine mammals from the service's exercises.

Ginsburg said that "this likely harm ... cannot be lightly dismissed, even in the face of an alleged risk to the effectiveness of the Navy's 14 training exercises."

Roberts pointed out that the federal appeals court decision restricting the Navy's sonar training acknowledged that the record contained no evidence marine mammals had been harmed.

The exercises have continued since the 9th U.S. Circuit Court of Appeals in San Francisco ruled in February that the Navy must limit sonar use when ships get close to marine mammals.

A species of whales called beaked whales is particularly susceptible to harm from sonar, which can cause them to strand themselves onshore.