RIO BRANCO, BRAZIL, DEC. 15 -- Rancher Darly Alves da Silva and his son Darcy were found guilty tonight of killing ecologist and union organizer Chico Mendes, who had opposed their plans to raze tracts of Amazon rain forest to make pastureland.

Mendes was internationally celebrated for his efforts to defend the rain forest, and the traditional rural workers who depend on it for their survival, from rapid clearing and development by ranchers. He was killed Dec. 22, 1988, as he stepped out the back door of his tiny wood-frame house in the town of Xapuri. The slaying and the trial focused worldwide attention on this remote corner of the Amazon and the violent conflicts being waged over how the rain forest is to be used -- and how much of it is to be preserved.

A seven-member jury deliberated for three hours before delivering its verdict around 11 p.m. The courtroom, packed with Mendes' supporters, burst into applause.

Both men were sentenced to 19 years in prison. The jury found, in effect, that the two men had conspired to kill Mendes and had shot him in cold blood.

Darcy Alves, 23, stunned the tiny Xapuri courthouse on the opening day of the trial Wednesday when he confessed to killing Mendes. Defense lawyers argued that he was defending the honor and financial interests of his father, whose land-clearing Mendes had opposed, but they contended that the elder Alves did not know of his son's plans.

But the prosecution maintained that the entire Alves family was involved. As the crux of their case, prosecutors argued that Darcy Alves killed Chico Mendes because his father Darly told him to.

The Parana Ranch outside Xapuri, of which 56-year-old Darly Alves was lord and master, was portrayed by the prosecution's chief witness as a kind of wild-west badman's hideout, peopled by pistoleiros who settled their disputes by eliminating their enemies.

The witness, 15-year-old Genesio Ferreira da Silva -- who had lived at the ranch since he was a young child -- testified that he heard Darly Alves, his son and other family members and ranch hands plotting Mendes' murder.

Ferreira testified that he heard Darcy Alves come home the night of the killing in the company of a trusted ranch hand -- a key point that contradicted Darcy Alves' claim to have acted alone. Upon being told that Mendes was dead, Ferreira said, patriarch Darly Alves ordered that a cow be slaughtered and a barbecue held in celebration.

The youth, who has received death threats and now lives in another state, also told the court he knew of at least seven other killings carried out by Alves family members or ranch workers. Two Bolivian men were killed so the family could steal the cocaine they were trying to smuggle into Brazil, Ferreira said. A union organizer was killed, Ferreira testified, because Darly Alves thought him "a flunky for Chico Mendes."

Conflicts between ranchers and rural workers are common, and often deadly, in Brazil. Mendes, who headed the Rural Workers Union of Xapuri, championed the cause of the rubber tappers whose livelihood depends on leaving the rain forest largely intact. Ranchers seek to clear away forest, selling the valuable hardwoods to logging interests and ending up with pasture for grazing cattle.

In the past, ranchers had another incentive to clear land -- Brazilian government subsidies. The funds were only available if there was clear title to the land, and the surest way to establish title was to strip the land bare and put fences around it.

Labor leaders claim that more than 1,000 rural union activists have been killed over the past decade, and that only in a handful of these cases have the accused killers been brought to trial. The Mendes case was thus seen by some as an important test case of the Brazilian justice system's ability to mediate land disputes, which ecologists consider fundamental to the future of the Amazon.

The four-day trial was long by Brazilian standards. In Brazil, most fact-finding is done through depositions taken in advance, witnesses are kept at a minimum and the judge does all the questioning. Lawyers for the prosecution and defense spent today's entire session in their closing argument, using more time to cite legal points rather than review the facts of the case.

Just after sunrise today, friends and supporters of Mendes marked what would have been his 46th birthday. They walked out to the Xapuri cemetery where he is buried, lit candles at his grave and said a Mass. Later they gathered at his house, now the headquarters of the Chico Mendes Foundation, to lay a plaque at the site where he died.

"He will always live in our memories," said Mendes' widow Ilzamar.