RIO DE JANEIRO, FEB. 4 -- A rural labor union leader was shot dead in central Brazil Saturday, rekindling a bloody dispute over the region's fertile farmlands, human rights activists said today.

Violent land disputes stretch across Brazil from the Amazon rainforest in the north to the grasslands of the south. A killing that drew international attention, that of rubber tappers' union leader and environmentalist Chico Mendes, resulted in the conviction of two ranchers in December. It was one of the few cases resolved.

In the latest attack, Expedito Ribeiro de Souza, 43, president of the farm workers' union in Rio Maria, a cattle town 1,500 miles northwest of here, was shot three times in the head by an unidentified gunman -- the fifth land-related death there in the last year. Souza's union had aided small farmers battling for land in an area dominated by large ranchers.

In a report released Sunday, the human rights monitoring group Americas Watch said Souza, a member of Brazil's Communist Party, believed he was a marked man. Last April, a parish priest in Rio Maria had asked the minister of justice to provide protection for Souza.

"All too often," the Americas Watch report said, "violence is the quick and dirty way for rural elites to dispose of community and union 'troublemakers' who dispute their right to maintain thousands of acres of land uncultivated."

The Americas Watch report found that of 1,566 killings of farm workers, church people and other activists in land disputes in Brazil from 1964 through 1989, only 17 trials had been conducted and only 8 convictions achieved.

"Impunity for the perpetrators of targeted assassinations in rural Brazil continues to be the rule," the report concluded.

Souza's predecessor as president of the Rio Maria union, Joao Canuto, was killed in 1985. The police never completed the investigation.

Two of Canuto's sons, Joao and Paulo, were shot to death in April 1990. A third, Orlando, escaped the kidnappers and later identified them as members of a gang of gunmen linked to local landowners. Four suspects are awaiting trial.

Orlando Canuto, before fleeing Rio Maria, said his brothers' killers had said Souza was next on their list.

"He wasn't directly involved in any land conflict. He was simply the president of the union," said Olga Pereira dos Santos, member of a Roman Catholic human rights group in the state of Para.