Prisoners do not have a constitutional right to DNA testing after their conviction, the Supreme Court ruled yesterday, even though the technology provides an "unparalleled ability both to exonerate the wrongly convicted and to identify the guilty."

In the court's first examination of how to treat the rapidly evolving field of biological testing, Chief Justice John G. Roberts Jr. wrote for a majority that said it is up to the states and Congress to decide who has a right to testing that might prove innocence long after conviction.

The "challenges DNA technology poses to our criminal justice systems and our traditional notions of finality" are better left to elected officials than federal judges, Roberts wrote for the majority in a 5 to 4 decision.

"To suddenly constitutionalize this area would short-circuit what looks to be a prompt and considered legislative response," he wrote.

The case was one of the most anticipated of the term, given the revolutionary role DNA testing has played in modern criminal proceedings. The Innocence Project, a group that represents those who say they have been wrongfully convicted, said such testing has exonerated 240 people nationwide, at least 17 of whom had been sentenced to die.

Dissenting justices, led by John Paul Stevens, said the right to post-conviction DNA testing should not depend on the widely varying laws enacted by the states. Allowing a prisoner to test DNA evidence at his own expense would "ascertain the truth once and for all," Stevens wrote.

"On the record before us, there is no reason to deny access to the evidence and there are many reasons to provide it, not least of which is a fundamental concern in ensuring that justice has been done," he wrote. He was joined in dissent by Justices David H. Souter, Ruth Bader Ginsburg and Stephen G. Breyer.

The case at hand comes from Alaska, one of three states without a law allowing post-conviction access to biological evidence.

William G. Osborne was convicted of the brutal rape and assault of a prostitute in a secluded area near Anchorage International Airport in 1993. Osborne wanted to pay for a more discerning test of semen found in a condom at the crime scene, a test prosecutors agreed would almost definitively prove his guilt or innocence. But prosecutors refused to allow it, and Alaska courts agreed that Osborne did not qualify under the procedures they had established.

Osborne appealed to the federal courts, and the U.S. Court of Appeals for the 9th Circuit in San Francisco recognized a right to such testing under the due-process clause of the Fifth Amendment.

But Roberts, joined by Justices Antonin Scalia, Anthony M. Kennedy, Clarence Thomas and Samuel A. Alito Jr., said that was wrongly decided. "A criminal defendant proved guilty after a fair trial does not have the same liberty interests as a free man," he wrote, and thus states have more "flexibility" in deciding procedures for post-conviction relief.

The majority said Alaska's procedures seemed sufficient and there was no reason for federal courts to "leap ahead" of the states.