There is potential in the near future in the northern Great Plains for a cellulose-based ethanol industry. Feedstock provision will require high-yielding cultivars of native warm-season grasses. The objective was to determine genetic variation for biomass production within 2 cultivars (Summer and Sunburst)of switchgrass (Panicum virgatum) and among 7 regional populations of prairie cordgrass (Spartina pectinata)at 2 locations in eastern South Dakota. Averaged across 4 years, family means for biomass harvested in early October ranged from 2.7 to 6.3 Mg DM/ha for Sunburst and from 4.3 to 7.2 Mg DM/ha for Summer. The range was from 5.0 to 7.9 Mg DM/ha for the prairie cordgrass populations. Significant differences among families indicated additive genetic variation within cultivars of switchgrass, and significant differences among populations indicated genetic variation within prairie cordgrass for biomass production in the northern Great Plains.