Rescue teams near Cape Town have managed to return 20 stranded pilot whales to the water.
But 30 others which beached at 0530 GMT on Saturday had to be put down.
Bulldozers were used to try to push the 55 adults and calves back into the water. However, the operation was hindered by high winds and waves. Rough seas pushed some whales back ashore.
The BBC reports those which could not be saved were shot humanely in the head.