If he keeps his word, Castro will leave office no later than 2018. Cuban-American exiles in the United States have waited decades for the end of the Castro era, although they will likely be dismayed if it ends on the brothers' terms.
Nevertheless, the promise of a change at the top could have deep significance for U.S.-Cuba ties. The wording of Washington's 51-year economic embargo on the island specifies that it cannot be lifted while a Castro is in charge.
In Florida, home to hundreds of thousands of Cuban exiles, some were skeptical that Castro's eventual retirement will change much.
"First we have to see if he lives another five years, and after we have to see what happens," said Raul Lopez Mola, an 81-year-old who abandoned Cuba in 1966 for a new life in Miami. "No one can predict what will happen in five years. For me, I don't think it has great importance."
"It would be more meaningful if Fidel Castro died," Lopez Mola added.
Fidel Castro is 86 and retired, and has appeared increasingly frail in recent months. He made a surprise appearance at Sunday's gathering, receiving a thunderous ovation from lawmakers.
Some analysts have speculated that the Castros would push a younger member of their family into a top job, but there was no hint of that Sunday.
While few things are ever clear in Cuba's hermetically sealed news environment, rumblings that Diaz-Canel, an electrical engineer by training and ex-minister of higher education, might be in line for a senior post have grown.
In recent weeks, he has frequently been featured on state television news broadcasts in an apparent attempt to raise his profile.
He also traveled to Venezuela in January for the symbolic inauguration of Hugo Chavez, a key Cuban ally who had been re-elected president but was too ill to be sworn in.
The 612 lawmakers sworn in Sunday also named Esteban Lazo as the National Assembly's first new chief in 20 years, replacing Ricardo Alarcon.
Lazo, who turns 69 on Tuesday, is a vice president and member of the Communist Party's ruling political bureau. Parliament meets only twice a year and generally passes legislation unanimously without visible debate.
The legislature also named as vice presidents of the ruling Council Machado Ventura; comptroller general Gladys Bejerano; second Vice President Ramiro Valdes; Havana Communist Party secretary Lazara Mercedes Lopez Acea; and Salvador Valdes Mesa, head of Cuba's labor union.
Associated Press writers Anne Marie-Garcia and Paul Haven in Havana, and Christine Armario in Miami, contributed to this report.
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