WASHINGTON — Passengers stopped in cars by the police are “seized,” the Supreme Court said yesterday, and therefore have a right to contest the legality of the stop if they are searched and arrested.

Justice Souter said “any reasonable passenger” of a car stopped by the police would understand the need not to depart unless given permission by the officer.

The 9–0 ruling, which clarifies the law on traffic stops, overturns the view of state courts in California, Colorado, and Washington, which have ruled that the passenger has no constitutional right to challenge the propriety of the initial traffic stop.

Whether the passenger in the case before the court yesterday will succeed in such a challenge will have to be determined by further litigation.

Typically, when officers stop a car, they question the driver. They may also inspect the passengers to make sure they do not have weapons. On some occasions, these traffic stops turn into drug arrests when officers find drugs or weapons on a passenger.

Yesterday’s ruling is likely to prove significant, however, only in cases in which police did not have a valid reason for stopping the car in the first place.

The case began in November 2001 when a police officer in Yuba City, Calif., saw what he thought was an expired registration tag and pulled over a Buick driven by Karen Simeroth. His suspicion was unfounded, and the temporary tag was valid.

The front-seat passenger was Bruce Brendlin, whom the officer recognized. The officer asked Mr. Brendlin to get out of the car and, after patting him down, discovered syringes and a plastic bag with a green leafy substance.

That triggered a further search of the car, and Mr. Brendlin was arrested and later convicted of making methamphetamine. The driver was not charged with any offense.

In court, Mr. Brendlin challenged his arrest as the product of an “unconstitutional seizure” by the police. He lost before the California Supreme Court, which ruled that a passenger in a stopped car “is not seized as a constitutional matter” and has no grounds for challenging the initial stop.

Last year, Mr. Brendlin appealed to the Supreme Court.

The Fourth Amendment forbids “unreasonable searches and seizures” by the government, and the Supreme Court in the past had said persons are seized by authorities when they are stopped, and they are not free to leave.

In yesterday ‘s decision, the court made clear what it had strongly implied in the past. “When a police officer makes a traffic stop, the driver of the car is seized within the meaning of the Fourth Amendment,” Justice Souter said. “We hold that a passenger is seized as well and so may challenge the constitutionality of the stop. … Brendlin was seized from the moment Simeroth’s car came to halt on the side of the road.”

Justice Souter noted, however, that his opinion dealt only with private cars. A different rule may apply to taxi cabs or buses, which passengers may well feel free to leave if an officer stops and questions the driver.

Separately, the Supreme Court threw out an antitrust case against Wall Street investment banks accused of having conspired to drive up prices on 900 newly issued stocks during the dot-com bubble of the late 1990s.

The justices, in a 7–1 decision reversing a federal appeals court ruling, held that securities law held sway over antitrust laws. Only Justice Thomas ruled against the investment banks, led by Credit Suisse.