The Supreme Court yesterday reversed the murder conviction of a black Louisiana death-row inmate on the grounds that racial bias had infected the selection of his jury.

The 7-2 decision is the court's latest effort to press trial judges to intervene when a prosecutor moves to exclude blacks from the trial of a black defendant.

In yesterday's opinion, the court said a trial judge in Jefferson Parish, La., "committed clear error" by sitting idly while prosecutor James A. Williams excluded all the blacks in the jury pool for the 1996 trial of Allen Snyder, an African-American accused of stabbing to death a man his estranged wife was dating and wounding her.

The same prosecutor also referred to the trial as "his O.J. Simpson case" because, he said, the facts were "very, very similar" to the famous murder case in Los Angeles.

The court has "resoundingly told judges and prosecutors throughout the country that the practice of striking people from jury service based on their race must end," said Atlanta civil rights lawyer Stephen Bright, who represented Snyder. "I hope that, as a result of this decision, juries will be more representative of their communities."

Though it is commonly said that Americans have a right to trial by a jury of their peers, the Constitution does not use those words. It says only that defendants have a right to a "public trial by an impartial jury."

Nearly a quarter of the population of Jefferson Parish is black, yet a study of death penalty cases tried there found that many were decided by all-white juries.

For its part, the Supreme Court has said that excluding jurors because of their race violates the Constitution's guarantee of the equal protection of laws. Enforcing that rule is not easy, however, because it is not entirely clear why a prosecutor may choose to remove a particular juror.

Both the prosecutor and the defense lawyer have considerable freedom to shape the jury. Both sides may remove potential jurors for cause - for example, jurors may be excused because they know or work with someone involved in the case.

In addition, both sides are free to exclude a certain number of jurors for any reason, including a hunch about their views. In Louisiana, each side can use up to 12 of these "peremptory challenges."

In Snyder's case, all five of the prospective black jurors were eliminated by the prosecutor.

Louisiana's highest court had upheld Snyder's conviction in a 4-3 decision.

In overturning the conviction, Justice Samuel A. Alito Jr. described the prosecutor's reasons for excluding the last black juror as suspicious and implausible. The contrast of that action with the decision to retain a white juror with a busy schedule was "particularly striking," he added.

When put together, the evidence strongly suggests the prosecutor was "motivated in substantial part by discriminatory intent" when he shaped the jury, Alito said. His opinion focused narrowly on the facts of this case, and did not lay out a new or clear principle of law.

Justices Clarence Thomas and Antonin Scalia dissented. Thomas faulted the majority for second-guessing the prosecutor's "race-neutral reasons" for excluding the black jurors, including the college senior.