The 32-seat rickety bus rolled into the ferry and the driver was unable to stop it due to apparent break failure. It broke through the poor safety rope and fell directly into the Rangamatia river in southwestern Barisal, local people and survivors said.
Divers found no body inside the bus, while rescuers feared the strong current in the river near the Bay of Bengal has washed away the victims.
Rescue efforts have also been hampered due to the lack of daylight.
The survivors and witnesses said many passengers, mostly women and children, were sitting inside the bus when it fell off the ferry.