Eight experiments investigated how different sowing and harvest dates, plant populations and fertilizers affect yield, germination and size of seed from hybrid monogerm sugar-beet seed crops, grown in situ when raised under barley cover in Lincolnshire, or open sown in Oxfordshire. Weather at seed harvest was wetter than average in 1968, average in 1969, and drier in 1970. Most seed of good quality came from plant populations of 300000 plants/ha sown in early July, given ample P and K fertilizer in the seed bed and 150–200 kg/ha of N in the spring of the second year in Oxfordshire or 200–250 kg/ha N in Lincolnshire. Harvesting in early September on average yielded most seed. Methods used to grow multigerm seed proved suitable also for monogerm seed but the percentage germination was lower for monogerm than multigerm seed.