A survey of cut seed lots from Klamath Basin commercial potato operations showed poor uniformity of seed piece size and numerous undersize seed pieces in most of the 18 seed lots examined over two years. Experiments were conducted in 1995, 1996, and 1997 to evaluate effects of seed piece size on the performance of Russet Burbank, Russet Norkotah, and Century Russet cultivars. Weighed seed tubers were cut into four pieces to obtain seed piece sizes of 21, 35, 50, 64, and 78 g (+/-2 g). Seed pieces less than 50 g resulted in delayed emergence and reduced stem numbers in all varieties. Effects of seed piece size on yield and tuber size distribution were greatest for Century Russet and least for Russet Norkotah, but were significant for all varieties over the three-year study. Increasing seed piece size increased yield of U.S. No.1 and total yield, while reducing tuber size. An economic analysis, based on crop values for fresh market use and seed costs, found optimum seed piece sizes of 64 g for Russet Burbank and Century Russet and 50 g for Russet Norkotah.