As part of a larger effort to study the fate and transport of agrichemicals in the midwestern United States, the U.S. Environmental Protection Agency commissioned a study of spatial variability within the Western Corn Belt Plains ecoregion. The objective of this study was to systematically partition the study area into subregions that best reflected the ecological diversity of the region. A process was developed which relied to a large degree on small-scale digital data to quantify spatial concordance among data sets, group data sets into related themes, assign boundaries based on spatially-variable contingency statements, and reflect the uncertainty in transition zones between subregions using variable width boundaries. Some of the most prominent ecological units were identified on the basis of surficial geology, soils, vegetation, and land use. The systematic methodology used offers advantages over some previous methods for identifying ecological regions.