This paper reports on the development of a unidimensional genetic knowledge index that has been tested and validated in a general population sample. The Index is intended to provide the basis for a standard measure of basic genetic knowledge that can be applied across diverse populations and research settings. The study group was composed of 330 European Americans selected randomly in the Louisville, KY, metropolitan area. The final version of the Genetic Knowledge Index (GKI) consisted of five items identified by principle components analysis, correlation coefficients, and the alpha measure of internal consistency. Construct validity of the GKI was determined by appropriate statistical correlations with educational attainment and attitudes toward genetic discrimination. The Index provides a numerical ranking of subjects' knowledge of practical genetics. Implications for research are discussed.