This data set consists of digital polygons of constant hydraulic conductivity values for the Tillman terrace and alluvial aquifer in southwestern Oklahoma. The Tillman terrace and alluvial aquifer encompasses the unconsolidated terrace deposits and alluvium associated with the North Fork of the Red River and the Red River in the western half of Tillman County. These sediments consist of discontinuous layers of clay, sandy clay, sand, and gravel. The aquifer extends over an area of 285 square miles and is used for irrigation and domestic purposes. Granite and the Hennessey Formation outcrop in northern parts of the aquifer where alluvial deposits are absent. These outcrops were included as part of the aquifer in a thesis in which the ground-water flow in the aquifer was modeled. An average hydraulic conductivity value of 92.5 feet per day was used for both the terrace and alluvial deposits in this data set and was reported in the thesis. The hydraulic conductivity polygons were derived from two sources. The outer polygon representing the outer shell of a model grid for a ground-water flow model of the Tillman terrace and alluvial aquifer was digitized from a paper map in a thesis at a scale of 1:249,695. Polygons and lines representing geologic contacts were extracted from a published digital surficial geology data set based on a scale of 1:250,000. Small polygons along the eastern boundary of the aquifer were created when the outer polygon of the model grid and the geology polygons and lines were combined. These small polygons represent geologic units other than the Tillman terrace and alluvial aquifer within the model grid. Three small polygons representing outcrops of granite and the Hennessey Formation in the northern parts of the aquifer also were extracted from the digital surficial geology data set. Ground-water flow models are numerical representations that simplify and aggregate natural systems. Models are not unique; different combinations of aquifer characteristics may produce similar results. Therefore, values of hydraulic conductivity used in the model and presented in this data set are not precise, but are within a reasonable range when compared to independently collected data.