A bus bound from Miami to Atlanta made a stop in Fort Lauderdale, and two Broward County sheriff's department officers boarded.

For police activity to constitute a seizure, the Court had held in Terry v. Ohio, 392 U.S. 1 (1968), that there must be a show of physical force or other authority.

And in Florida v. Royer, 460 U.S. 491 (1983), the Court had remarked that the police could approach a suspect in a public place (there, an airport concourse) and ask him a few questions without violating the Fourth Amendment.

The question in Bostick was whether the fact that the police approached the defendant while he was a passenger on a bus, by itself, rendered the encounter "unreasonable" under the Fourth Amendment.

In Michigan v. Chesternut, 486 U.S. 567 (1988), the Court had suggested that a seizure occurs whenever a reasonable person does not feel "free to leave" an encounter with the police.

In the absence of such a show of authority, there was no justification for the Florida court's per se rule that a seizure had occurred simply because the encounter had taken place on a bus.

"Rather than requiring the police to justify the coercive tactics employed here, [Justice O'Connor] blames respondent for his own sensation of constraint....

Thus..., because respondent's freedom of movement was restricted by a factor independent of police conduct - i.e., by his being a passenger on a bus - [Bostick] was not seized for purposes of the Fourth Amendment."