Rescuers have been trying to save a pod of around 60 long-finned pilot whales in the Kyle of Durness, a sea inlet near Cape Wrath.
Around 35 beached as the water receded at low tide and 20 were refloated using inflatable pontoons as the water level rose again.
However, the remainder perished, said the British Divers Marine Life Rescue (BDMLR) charity.
The rescue effort was stood down following nightfall Friday and was to resume after sunrise Saturday.
Mark Simmonds, head of science at the Whale and Dolphin Conservation Society, said the Kyle of Durness looked like a "whale trap".
"Something may have startled the group further out to sea and they panicked, came into this unusual situation and were unable to find their way out.