An ecologically based classification system, when accompanied by digitized maps of biotic communities, has been shown to be useful for plotting and assessing affinities of plants and animals in the southwestern United States and northwestern Mexico. Because these maps show ecological relationships of plants and animals with their environment, maps of biotic communities can be especially informative when delineating and describing affinities of habitats. We have expanded the classification system and prepared a digitized, ecologically based, color map of the biotic communities of North America to assess distributions of organisms on a continental as well as regional scale. As such, this map also can be used as a sample frame to design and stratify surveys of animals.