WASHINGTON -- Convicts have no constitutional right to DNA testing, the Supreme Court found, a ruling that left the accessibility of potentially critical evidence to the discretion of individual states.

In a 5-4 vote along the court's ideological divide, the conservative majority acknowledged that "modern DNA testing can provide powerful new evidence unlike anything known before." But the opinion, by Chief Justice John Roberts, drew a sharp distinction between defendants, who are presumed innocent at trial, and convicts like...