XAPURI, BRAZIL, DEC. 14 -- The prosecution's key witness described today how he overheard rancher Darly Alves da Silva and his son Darcy plan the murder of union organizer and ecologist Chico Mendes and celebrate Mendes' death.

The testimony by 15-year-old Genesio Ferreira da Silva was seen as crucial to the prosecution's contention that Darly Alves, 56, knew and approved of his son's plan to kill Mendes. Darcy Alves, 23, has confessed to the crime but maintains his father was not involved.

Mendes was shot dead on Dec. 22, 1988, as he stepped out the back door of his house. Mendes had won a United Nations award and had been praised by ecologists worldwide for his work in organizing rural workers to fight destruction of the Amazon rain forest, upon which thousands of rubber tappers depend for their precarious livelihood.

Ferreira, who said he is related to the Alves family by marriage, had lived at Darly Alves' ranch compound near here since he was a young boy. In almost a full day of testimony he chillingly, matter-of-factly portrayed the Parana Ranch as a world where murder was almost routine and being caught was extremely unlikely.

He ascribed seven other killings to Alves family members and ranch hands. The victims, he said, had espoused the wrong political views, offered some personal insult or just happened to be in the wrong place at the wrong time.

But there were inconsistencies in Ferreira's testimony, and he acknowledged that much of what he knew about the Alves family's alleged crimes came from what he overheard or what others told him, not from what he saw.

Ferreira, a slight youth who spoke slowly but with assurance, said that before the killing he had heard the Alves family "planning the death of Chico Mendes. . . . They were saying there was going to be a stakeout." That point, however, was not developed further in today's questioning.

The defendants were not in the courtroom when the youth testified.

On the night Mendes died, Ferreira said, he heard Darcy Alves return to the ranch along with a ranch hand named Serginho and announce to his father: "The man is dead."

Ferreira said he was in bed at the time but could hear the voices of Darly Alves, Darly's brother Alvarinho, Darcy Alves, Serginho and three others who lived at the ranch.

Darly Alves asked who had pulled the trigger, according to Ferreira, and Darcy Alves responded that he had fired the shot.

"The cow is already in the pen," Darly Alves told his son and the others, according to the witness. "Tomorrow we will have a barbecue."

In his unexpected confession during Wednesday's opening session of the trial, Darcy Alves said he committed the crime alone. The testimony today supported the prosecution's contention that Serginho went along, which is intended to bolster the idea that the whole group of family members and hired hands helped plan Mendes' death.

Serginho, known as Sergio Pereira or Antonio Pereira, fled shortly after the killing and has not been captured.

Ferreira told of how the family had its planned barbecue the next day and how Darly Alves and several other family members and ranch hands packed their belongings and fled. Ferreira was arrested two days after the Mendes killing.

His version of events was inconsistent at several points. He was particularly unsure of the day of the week on which the killing took place, and he omitted several alleged crimes by the Alves family that he had listed in an earlier deposition taken by police.

Hundreds of union organizers and other activists have been killed over the past 15 years, and the vast majority of those cases remain unsolved. The basic issue in most disputes is land use, with ranchers pitted against landless workers, rubber tappers and others who oppose wholesale clearing of the Amazon rain forest.

At the time of his death, Mendes was successfully blocking the Alves family's attempt to clear a large tract of forest. His case is seen by many as a test that will show whether Brazilian justice is able to confront violence and impunity.

Ferreira described the following incidents -- mostly without dates or other details, and all without corroborating witnesses:

Two Bolivians walked into the Alves ranch, which is less than 50 miles from the Bolivian border, and asked for water. Darcy and one of his brothers believed the bags they were carrying might contain cocaine, so they and several ranch hands followed the men when they left. Ferreira heard two shots and later saw the men's bodies dumped along a road inside the ranch.

A ranch hand named Raimundo was killed because he had asked for one of Darly Alves' daughters in marriage. Darly Alves had decreed that the girl "was going to marry a man, not a peon."

A youth named Zeca stole some herbicide and sold it to Darly Alves. He was killed because the family believed police on Zeca's trail might eventually be led to the Parana Ranch and uncover other crimes.

A union member and candidate for the town council, Ivair Higino, was killed because Darly thought him a "flunky for Chico Mendes."