Juanita Castro, 76, initially supported her brother's 1959 overthrow of the Batista dictatorship but quickly grew disillusioned. In a Spanish-language memoir published by Santillana USA and cowritten by journalist Maria Antonieta Collins, she says the wife of the Brazilian ambassador to Cuba persuaded her to meet a CIA officer during a trip to Mexico in 1961.

By then, her house had become a sanctuary for anticommunists, and Fidel Castro had warned her about getting involved with the gusanos, or worms, as those who opposed the revolution were called.

Juanita Castro says in the book My Brothers Fidel and Raul: The Secret Story that she traveled to Mexico City under the pretense of visiting a younger sister, Enma. There she also secretly met a CIA officer who identified himself as "Enrique" at the elegant Camino Real hotel.

The book identifies her CIA contact as Tony Sforza, one of the spy agency's Cuba experts and a member of the post-Bay of Pigs campaign against the Castro government. He had been in Cuba passing himself off as a gambler named Frank Stevens.

A spokesman for the CIA in Langley, Va., declined to comment on her account.

Juanita Castro says that during the hotel meeting, she expressed her concerns that those who supported Batista's overthrow but were not communists were being pushed out of the new government. She writes that she agreed to help the CIA gather information but refused to accept money for her efforts and said she wanted no part in any violence.

"I want to be very clear that agreeing to collaborate with you does not signify that I will participate in any violent activity against my brother, nor any official in the regime," she told the agent. "This is my most important condition. And moreover, I would say it is the only condition."

"Enrique" then asked her to smuggle messages, documents, and money back into the country hidden in canned goods. He told her she would receive information through shortwave radio communications. She chose a waltz and a song from the opera Madame Butterfly as the signals her handlers would use to let her know if they had information for her.

She said she remained on the island while her mother was alive, believing she was protected from the full wrath of Fidel. Her mother died in 1963 and she fled Cuba the following year, eventually settling into a quiet life in Miami, where she ran a pharmacy until 2007 and is generally well-regarded by other Cuban exiles.

Juanita Castro wrote that Fidel was not initially a hard-line communist like their brother Raul and fellow revolutionary Ernesto "Che" Guevara but that Fidel turned to communism to maintain power. She said she tried to help many people who initially supported the revolution only to be ousted in the new regime's initial purges.

"My brothers could ignore what I did, or appear to ignore it, so as not to hurt my mom, but that didn't mean I didn't have problems. . . . Everything was becoming more dangerously complicated" after her mother's death, Juanita Castro writes.

She had to get help from Raul, to whom she was much closer than Fidel, in getting a visa to leave Cuba. They have not seen each other since June 18, 1964, the day before she left the country.

Later, under President Richard Nixon, the CIA told her it no longer supported the underground fight against Fidel Castro because it negatively affected U.S.-Soviet relations. She said the CIA wanted her to make statements that communism in Latin America was no longer a threat. At that point, she broke off with the agency, she said.