The 86-year-old former communist leader was seen voting at a polling station where he spent some time talking to other voters and to the media, Cuba's state-run media said.
Castro was stooped over and his voice seemed weak, the BBC said.
A crowd surrounded his car and cheered him as he left.
More than 600 delegates in the National Assembly will approve the candidates for Cuba's key political positions. With President Raul Castro, the younger brother of Fidel, already 81, the choice of vice presidents and ministers could provide an indication about who might be a successor, the BBC said.
All candidates were chosen by the ruling Communist Party or its affiliated associations.