The latest in a series of essays by the 80-year-old Castro, who has not been seen in public since becoming ill more than 10 months ago, was published on the front page of the Communist Party daily Granma.
The Cuban leader said that President Bush, asked recently about his Cuba policy, replied: "I'm a hard-line president and I'm only waiting for Castro to die."
"I'm not the first, nor will I be the last, who Bush has ordered to be deprived of life," said Castro, who offered no details of the alleged conversation.
American law now prohibits the U.S. government from ordering the assassination of foreign leaders, but declassified U.S. documents show that the CIA made numerous attempts to kill Castro in the early years after the 1959 Cuban revolution.
"Ideas are not killed," Castro wrote.
He criticized the Bush administration for spending on weaponry while people in developing nations go hungry.
"I ask myself how many doctors can graduate with the $100 billion that in just one year fall into Bush's hands to continue to sow mourning in Iraqi and American homes," he wrote. "The answer: 999,990 doctors, who could attend to 2 billion people who today receive no medical care."
Castro shocked Cuba on July 31 when he announced that he had undergone emergency intestinal surgery and was stepping aside provisionally for his younger brother Raul, the defense minister, during his recovery.
Although senior Cuban officials have said Castro is on the mend, it seems more unlikely with time that the bearded leader will return to power.
Castro's exact ailment and condition remain state secrets, but he is widely believed to suffer from diverticular disease, which causes sacs in the colon that can become inflamed and bleed.
CBS News Havana procuder Portia Siegelbaum reports Castro's latest editorial also serves as the ailing leader's commentary on the upcoming G-8 summit in Germany - or more acurately, another attack on President Bush, this time focusing on the chief topic of the summit; climate change.
He quotes the German Minister of the Environment as telling a French news agency it will be "very difficult" for the G-8 summit to successfully confront global warning due to United States' opposition.
President Bush, who Castro refers to by first name alone in the column, "had better decide what he really thinks in the G-8 meeting" on the dangers facing peace and the world's food supply, reports Siegelbaum.
Practically all of the commentaries he has written have been signed and dated at the end of what would be the regular work day for most people, between 5 and 8 p.m. Observers say that the regularity of his output and their timing suggests that for the first time in his life, Castro is following a normal schedule.
If he is reading, writing and undergoing physical rehabilitation during the day and sleeping at night as they suggest, it represents a complete break with his previous pattern of working round the clock and only catching a few hours sleep mid-morning.
"Castro could make a career as a syndicated columnist", says Siegelbaum.
The Castro statement was released as five U.S. lawmakers made an unannounced visit to Cuba on Monday, appearing at an agricultural trade fair during which the communist island hopes to sign contracts for as much as $150 million in American goods.
Rep. Rosa DeLauro, D-Conn., is heading the delegation, which plans to meet with at least one top Cuban official before returning to the United States, said Sarah Stephens, director of the Washington-based Center for Democracy in the Americas, which opposes the U.S. embargo toward Cuba and helped organize the trip.
Also in Cuba were Democratic Reps. Marion Berry of Arkansas, and Bob Etheridge of North Carolina, as well as Rep. Rodney Alexander, R-Louisiana, and Rep. Jack Kingston, a Republican from Georgia. All were making their first trips to the island — except Berry, who was here in 2000.
DeLauro, Berry and Etheridge have all supported legislation to ease U.S. trade restrictions toward Cuba in the past, while Kingston has supported the embargo. The lawmakers said agriculture trade opportunities were a key reason they came.
"We are a diverse group geographically and in our politics toward Cuba," DeLauro said. "But we view this as an opportunity to learn, to create dialogue about issues of mutual concern."
The trip coincided with a forum bringing together 114 food and agricultural companies from 22 U.S. states.
"This is really not a trade fair, this is a formal meeting to sign agreements with different companies that have been in progress for months," said John Kay, director of international trade for Alabama's Department of Agriculture. "It's an organized madhouse. You only have a certain timeframe to get everything done."
Pedro Alvarez, chairman of Cuba's food import company Alimport, said talks should produce enough deals to ensure the island buys as much U.S. goods in 2007 as it did last year, when Cuba spent $570 million for American food and agricultural products, including shipping and banking costs.