More than 100 dead Magellan penguins, coated in oil, have been washed up on the southern coast of Argentina.
Some 200 other penguins survived and are being treated and cleaned before being released.
Authorities have implemented an emergency plan with environmental workers patrolling the coastline on the lookout for more birds.
Meanwhile an investigation has been launched to establish where the oil came from.
Cape Virgenes, where the penguins were discovered, is a remote location in the far south of Argentina, a distance of some 2,400 km (1,350 miles) from Buenos Aires.
The survivors have been taken to a cleaning centre in the nearby town of Rio Gallegos.
More than 100 have been treated so far – the oil removed from their eyes and breathing passages, and their feathers cleaned.
Covered in oil, the penguins cannot tolerate the cold waters of the southern Atlantic, will not dive for food in the still-colder deep waters, and simply will not catch enough fish to survive.
While the penguin rescue operation is under way, an investigation has also been launched into where the oil came from – whether any petrol company has reported a leak, or an oil tanker has lost some of its load.
So far nothing has been reported.
Once a year, 80,000 Magellan penguins go to Cape Virgenes to nest, making it the second-biggest penguin colony in the world after Point Tombo further up the Atlantic coast.
As the southern hemisphere winter sets in, many penguins had already migrated to warmer areas, which the experts hope will keep the number of casualties to a minimum.
By Daniel Schweimler
BBC News, Buenos Aires