Tommaso Buscetta, the Mafia boss whose recent revelations to Italian investigators have sparked a dramatic crackdown on the underworld in Sicily, appeared in an open court here today for the first time since he decided to turn informer.

Buscetta, a 56-year-old Palermo drug trafficker, appeared briefly in a Rome court to testify about the 1982 murder in prison of another Palermo underworld figure, Pietro Marchese.

Responding to questioning by two judges from the Palermo Court of Assizes, the Mafia boss who was known to his cronies as "Don Masino" repeated his description of the Mafia as an organization run by an all-powerful "supercommission" of family bosses.

For Italian investigators, the appearance today in court by Buscetta, the most important Mafia member ever to have become an informer in Italy, was significant. Much of the success of Italy's crackdown on the Mafia will depend on his willingness to name names and to speak out publicly against his former allies and fellow Mafiosi.

Buscetta's original disclosures to Palermo magistrates investigating the Mafia led to the issuing of 366 warrants and more than 60 arrests in late September. Last week 127 more arrest warrants were published and 56 more arrests made.

Today, speaking in a special courtroom attached to Rome's Rebibbia Prison, Buscetta provided background to the killing of Marchese, who was stabbed 33 times in jail, allegedly because he deserted his Mafia clan, the Greco di Ciaculli family, for that of a rival.

"Yes, I know these things because I am a man of honor," said Buscetta, indicating that despite his decision to turn informer he still considers himself a member of the Mafia. In Mafia parlance, "man of honor" is synonymous for a ranking boss or figure.

Today, speaking before court officials, police and a handful of journalists, Buscetta said that the murder of Marchese could not have been carried out without the approval of Michele Greco, reputed to be the top Sicilian Mafia leader.

"The explanation is a bit complex, but in synthesis it is this. There are the families and there is a commission whose head is Michele Greco. The commission is formed by 10 heads of Mafia families," Buscetta said. He explained that there is one representative on the commission for every three Mafia families.

Asked if the Marchese murder was ordered by the commission, he said, "There is no doubt. It was decided by the commission. One cannot kill a man of honor just like that. Another family cannot kill him without provoking a war. Such a killing can be decided only by the head of the family to which the man of honor belongs or by the commission."