WASHINGTON -- The Supreme Court reinstated the drug trafficking conspiracy conviction of a St. Mary's County man yesterday, ruling that a Maryland appeals court had been "manifestly wrong" in ruling in his favor.

In an unsigned three-page ruling, the court overturned a decision a year ago by the Maryland Court of Special Appeals nullifying the conviction of Kevin Darnell Dyson. The justices' action was a victory for the Maryland attorney general's office, which appealed the case late last year.

Dyson had been sentenced to 10 years in prison, but was ordered to serve only 18 months. It is unclear whether he will have to return to jail because of the Supreme Court's action.

The state court found that a St. Mary's County sheriff's deputy, Sgt. Lyle E. Long, had reliable evidence from an informant's tip that Dyson would be carrying cocaine in a rented car in July 1996. But the court decided that was not enough to justify stopping the car, arresting Dyson and seizing the drugs because Long did not have a court-approved warrant.

If the officer had acted promptly to follow up on the tip, the arrest and the seizure of the drugs would have been justified without a warrant, the state court said. But Long had plenty of time -- perhaps 13 hours -- to get a search warrant, and the Constitution required him to have one in the absence of an emergency, it added.

Rejecting that approach as "demonstrably erroneous" and "manifestly wrong as a matter of federal constitutional law," the Supreme Court said the tipster's evidence was sufficient to support the arrest and seizure.

The court's vote was 7-2. The dissenters, Justices Stephen G. Breyer and John Paul Stevens, said the court should not have acted without hearing a defense of the state court ruling by Dyson's lawyer, Antoini M. Jones of Riverdale, or some other attorney.

The court had asked Jones to reply to the state's appeal, but said yesterday he did not. Jones said he offered in April to do whatever necessary but did not hear further from the court.