This biological reserve is an island off the Corcovado National Park on the Osa Penninsula. The island has an area of some 200 hectares but the reserve also includes 2,700 hectares of sea floor. The reserve was founded in 1978.

This island was an indian sacred burial ground. There are also indications that the island was an important trading point for the entire coast of this region. There are a number of stone spheres like those in Diques in mainland Costa Rica.

The island is covered by wet, tropical forest with trees of up to 50 meters, including specimens of the famous "cow tree", whose sap can be drunk and which was once used as emergency food for slaves.

There is not very much in the way of land fauna due to the small size of the island, but the park protects an impressive list of marine species.