The map of freshwater fish species richness - the number of species present in each ecoregion - was generated from a database constructed for this project. Fish data came from a variety of sources. For the United States, distribution data in the form of species assignments to 8-digit HUCs (hydrologic unit codes) were provided by NatureServe, and these were aggregated to freshwater ecoregions. For some small islands, species lists were generated using FishBase and then were augmented where possible with information from published literature. For most other ecoregions, species lists were provided by experts. For a small number of ecoregions it was impossible at this time to generate species lists, and richness estimates are provided instead; these ecoregions are shown with hatching. The numbers shown in this map represent only species using freshwater for at least a portion of their life cycles; these species were identified using the habitat assignments in FishBase. Extirpated species are included in these tallies, but resolved extinct species, as determined by the Committee on Recently Extinct Organisms (data provided by Ian Harrison), are excluded. Introduced species are not counted.