ASHINGTON, Feb. 28 - The Supreme Court heard arguments Monday on whether the Americans With Disabilities Act applies aboard passenger ships that call at United States ports while flying foreign flags - that is to say, nearly every cruise ship that serves the United States market.

The 1990 law guarantees to people with disabilities the "full and equal enjoyment" of any "place of public accommodation." The lower federal courts disagree on whether the definition includes the floating resort hotels that are particularly popular among people with disabilities, who select cruise vacations at a higher rate than the general population.

The United States Court of Appeals for the Fifth Circuit, whose jurisdiction includes the ports of New Orleans and Houston, ruled last year that the law did not apply, dismissing a suit filed against Norwegian Cruise Line by three passengers with mobility impairments and their two traveling companions.

The passengers claimed that they had been charged an unjustified premium for their accommodations on the Norwegian Sea and the Norwegian Star and that public restrooms and recreational facilities, including the swimming pools, had been inaccessible. The Justice Department supported the lawsuit.

The appeals court rejected the view of the Justice and Transportation Departments that the disability law applies to all cruise ships that enter United States ports, regardless of their country of registry. It is up to Congress to state clearly whether the law applies, and Congress has not done so, the Fifth Circuit said.

By contrast, the United States Court of Appeals for the 11th Circuit, which includes the ports of Fort Lauderdale and Miami, reached the opposite conclusion five years ago when it reinstated a lawsuit brought by a wheelchair-using passenger against a cruise line based in the Bahamas.

On Monday, Thomas C. Goldstein, arguing the appeal for the Norwegian Line's passengers, said the Fifth Circuit had misunderstood the legal principles governing the case. The plaintiffs are seeking not extraterritorial application of United States law, Mr. Goldstein said, but rather application of that law within sovereign United States territory. His clients have been subjected to discrimination "on the land, in the ports and on the waters of the United States," he said.

David C. Frederick, arguing for Norwegian Cruise Line, warned the court against applying a "Pandora's box of domestic legislation," including occupational safety laws and food and drug laws, to foreign ships. Federal judges would become the "special masters of the cruise industry," Mr. Frederick said. He said the cruise line denied the charges of discrimination, which remain untested because the case was dismissed before trial.

The cruise line, with a business office in Miami, is a subsidiary of Star Cruises, based in Hong Kong, and registers its ships in the Bahamas. Arguing for the government of the Bahamas on behalf of Norwegian, another lawyer, Gregory G. Garre, said that applying the disability law to foreign ships would invite "international discord and confusion."

An assistant solicitor general, David B. Salmons, joined Mr. Goldstein in arguing for the plaintiffs.

"Any vessel that comes into the waters of the United States and offers service to our residents" is covered by the Americans With Disabilities Act, Mr. Salmons said, adding that "the relevant question is whether the ship has offered a 'public accommodation' in the United States."

The justices took a lively interest in the case, Spector v. Norwegian Cruise Line Ltd., No. 03-1388. But they did not appear completely satisfied with either side's position, and it was unclear by the end of the argument what the eventual decision might be.

For example, Justice Ruth Bader Ginsburg told Mr. Goldstein: "You are in effect saying, 'The U.S. rules the world.' No matter what the other ports say, U.S. law is going to govern."

When Mr. Goldstein objected to that characterization of his argument, Justice David H. Souter rephrased Justice Ginsburg's point. "It rules the world unless the world doesn't want to use the U.S. as a port of call," he said.

Justice Ginsburg then took an equally stern tone with the cruise line's lawyer. Under Norwegian's position, she said, the public accommodations provision of the Civil Rights Act of 1964 would also be inapplicable to foreign ships, and "so a ship putting in at a U.S. port would be free to discriminate among the passengers on the basis of race."

Consistent with his legal position, Mr. Frederick, Norwegian's lawyer, had to agree, because Congress has not clearly stated that the Civil Rights Act should apply. He tried to address the question obliquely, saying, "Congress has not extended its laws to the full extent of U.S. power."

Justice Anthony M. Kennedy was not satisfied.

"We could write an opinion ruling for you but leave these other questions open?" Justice Kennedy asked in a skeptical tone. "I don't see how we can do that."

"This concerns me," Justice Ginsburg said. Observing that a majority of Norwegian's cruise passengers are Americans, she continued, "You're asking us to rule that an enterprise that is U.S.-centered is not bound by our bedrock antidiscrimination law."

When Mr. Frederick noted that compliance with the disability law on a ship could require structural modifications, Justice Antonin Scalia came to his assistance. "Why don't you draw that line?" Justice Scalia asked, explaining that the physical requirements of the statute could provide a reason for deciding that other civil rights laws applied on board ship while the disability law did not.

But Justice Sandra Day O'Connor found the reasoning unpersuasive. She noted that the disability law did not impose a blanket requirement for physical modifications, but only for those that are "readily achievable."

Whether requested changes were readily achievable could be decided later on a case-by-case basis, Justice Kennedy suggested.

Although the precise question before the court was a new one for the justices, the effort to define the reach of United States law to foreign ships is decades old and has proved quite difficult. Each side on Monday could invoke an inventory of precedents to support its position.

For example, a Prohibition-era decision by the justices held that in deference to United States law, foreign ships could not carry liquor when calling at United States ports, even if the liquor was kept under lock and key.

More recently the Supreme Court refused in 1963 to apply federal labor law to disputes between a foreign ship and its crew, on the ground that such disputes affected only the ship's internal management and not its behavior toward Americans. On the other hand, in a 1970 case, the court held that federal labor law covered disputes over wages paid by foreign ships to longshoremen working in American ports.