This report contains the findings from a rapid biological assessment of the Grensgebergte and Kasikasima mountains of southeastern Suriname. Suriname is one of the last places on Earth where an opportunity still exists to conserve huge tracts of pristine, diverse tropical forests. This volume is part of a series of RAP surveys in Suriname designed to support the protection of biodiversity, freshwater and other ecosystem services through collection of baseline biological and socio-economic data. The researchers in southeastern Suriname found over 800 species of plants, mammals, birds, reptiles, amphibians, fishes, beetles, ants, and katydids, and measured water quality. Of these, 60 species, including frogs and fishes, are probably new to science. A chapter also focuses on the overall importance of southeastern Suriname and the benefits that its pristine headwaters provide to people.