We develop a long-run cost model for cellulosic biofuel production that accounts for locational differences in biomass production conditions. The cost model minimizes the per-gallon cost of biofuel when feedstock costs vary within local biomass-producing regions and plant size is determined by local feedstock supply and size economies. Applying the model, we estimate U.S. ethanol supply costs from both corn stover and switchgrass. Model results are used to identify the locations and specific plant sizes from which stover-and switchgrass-based ethanol would meet cellulosic biofuel targets at least cost.