For three weeks, in November and December of 2000, Conservation International’s Rapid Assessment Program collaborated with a group of fourteen scientists-from Venezuela, Colombia, Brazil, and the United States-to survey the aquatic ecosystem of the Caura River in Venezeula. The area, part of the Guayana Shield, represents one of the most pristine watersheds in South America, due in large part to the excellent stewardship of its principle indigenous group, the Ye'kwana. It nonetheless faces a number of threats, ranging from a major hydroelectric dam and increased fishing pressure to rises in both tourism and agriculture. This assessment documents the Caura River Basin’s aquatic biodiversity, with pleasing results, including the tripling of the number of fish species known by science in the upper Caura and the identification of almost 400 species of plants, 87 species of aquatic insects, and four species of crab. As always, the report includes several recommendations for conserving this ecologically significant area.