Clear Lake in Lake County, California, has an endemic fish fauna composed of five lake-adapted forms derived from lowland stream-adapted forms present in surrounding drainage basins. Two of the five endemic forms are extinct. The three remaining endemics maintain themselves despite the destruction of sloughs and tule beds surrounding Clear Lake that are used for spawning and nursery areas. Trophic specializations of the endemic fishes indicate past selection for feeding on small benthic and pelagic invertebrates. The presence of fine particles in the substrate and the reduced activity of tributary streams for at least the past 10,000 yr are major...