BELEM, Brazil — A rancher pleaded not guilty yesterday to ordering the killing of American nun who died while trying to save the Amazon rain forest in a case that human-rights activists see as a test of Brazil’s commitment to halting violence over land.

Vitalmiro Bastos Moura is one of two ranchers accused of ordering the February 12, 2005, murder of Dorothy Stang, a naturalized Brazilian originally from Dayton, Ohio, in a conflict over land he wanted to log and develop.

“I had no participation whatsoever,” Mr. Moura, 36, told the judge in his opening statement, adding that he didn’t even know Stang, who had been active in organizing poor settlers around the jungle town of Anapu for the last 23 years of her life.

Stang, 75, was killed by six shots fired at close range on a muddy patch of road in the Amazon state of Para. She helped build schools and taught settlers to defend their rights and to respect the rain forest, earning the enmity of powerful men who hoped to exploit it.

A gunman, his accomplice, and a go-between have been convicted in the killing. The men, who are expected to testify, alleged that Mr. Moura and fellow rancher Regivaldo Galvao offered them $25,000 to kill Stang.

Wearing a black shirt and jeans, Mr. Moura defiantly told prosecutors he learned of the killing only after the gunmen fled to his ranch. He said he told them to leave and did not call police because doing so would only bring him trouble.

Mr. Moura also said he fled for 45 days shortly after Stang’s death because police gave him no chance to explain himself without being arrested.

Human-rights defenders said the trial could become a landmark case because of the difficulty of convicting the masterminds of crimes in Para.

In the past 30 years, 1,237 rural workers, union leaders, and activists have been killed in Brazilian land disputes. Of those killings, 772 took place in Para, and only four alleged mandantes, or masterminds, have stood trial. All four were convicted, but none is behind bars.

“This case is emblematic of the problems facing the Amazon region,” said Darci Frigo, a lawyer for the Brazilian group Land of Rights.

The case drew international attention and comparisons to the 1988 killing of the environmental activist Chico Mendes. Shortly after his killing, President da Silva ordered the army into the region, suspended logging permits, and ordered large swaths of rain forest off-limits to development.