Another Metrobus driver was shot and killed in the early hours of yesterday while driving near Faraday in Johannesburg.
The shooting led to the driver crashing the bus into a wall, seriously injuring a person.
Police spokesman Captain Cheryl Engelbrecht said it was not clear whether the driver was shot by a passenger from inside the bus or by someone outside.
She said witnesses apparently saw a black BMW speeding past the bus after hearing a gunshot.
"The driver was shot in the chest and the injured man was taken to Johannesburg Hospital in a serious condition," Engelbrecht said.
The driver, 51, is the second Metrobus driver to be shot while on duty since the beginning of the Metrobus strike last month.
Over a week ago a bus driver was shot in the abdomen in Kensington, Johannesburg. He is still recovering in hospital.
Bheki Shongwe, Metrobus managing director, said yesterday they could not rule out the possibility that the shootings could be related to the ongoing strike.
"We are conducting internal investigations regarding both shootings. We don't know if the incidents have anything to do with the strike at all. We are still gathering information and until then we will leave it in the hands of the law," he said.
The South African Municipal Workers Union (Samwu) has distanced itself from the shootings.
"We do not promote violence as a form of negotiation in Samwu," said the union's spokesman, Dumisani Langa.
"Our members are disciplined and would never force anyone to join our strike action when they do not want to participate."
The Metrobus strike started on January 31. Workers are in dispute with the employer over accumulated sick leave.