According to media reports, the ferry Barima was heading northwest along the coast, but sent out a distress call late Saturday evening local time.
At least 67 of the 133 people on board were reportedly rescued.
"Search and rescue efforts are progressing. 67 people, including 15 children, have been rescued so far," Juan Edghill, Guyana's Minister of Public Works, wrote on his Facebook page, AFP reports.
The ageing ferry is feared to have capsized after a large wave hit the ship, according to the minister.
